Mack Bros action build

[Halffull]

I am gathering parts for my first build. I just got my Mack Bros Evo II SS SA BDL action in hand. It looks great. I have a few observations to run past this crowd.

I mounted the action with trigger into a stock so I can cycle it a get the feel of it. I have been reading up on trigger timing and cock on close and so I measured this one. With a stock R700 trigger installed I am getting -.0255 coc and .2290 pin fall. Compare this to my R700 action with a TT trigger is at +.0330 coc and .2870 pin fall. So is negative coc good, bad or indifferent?

 

[D R A K E N B U R G]

From a accuracy and reliability point of view, a bit of positve cock on close vs negative cock on close, does not matter. What does matter is the amount potential energy in the spring.

I have no experience with the Mack Bros action, so assuming that it has reliable and accurate ignition, the reason why it can get away with it, despite the shorter pin fall, is probably because they have a stiffer or longer spring. They can thus store the same amount of potential energy in it compared to the Remington one.

The reason why most people talk about positive vs negative cock on close, is because of how it feels when the action is being cycled. What could be problematic however is when someone tries to modify an action that was designed to have a bit of positive cock on close to have negative cock on close. This could result in a striker spring that does not have enough potential energy. There are gun smiths that know how to do it properly, but is not as simple as one might think.

 

[Evintos]

I am gathering parts for my first build. I just got my Mack Bros Evo II SS SA BDL action in hand. It looks great. I have a few observations to run past this crowd.

I mounted the action with trigger into a stock so I can cycle it a get the feel of it. I have been reading up on trigger timing and cock on close and so I measured this one. With a stock R700 trigger installed I am getting -.0255 coc and .2290 pin fall. Compare this to my R700 action with a TT trigger is at +.0330 coc and .2870 pin fall. So is negative coc good, bad or indifferent?

Indifferent (preference) as long as there's enough pin fall for reliable ignition (firing pin tip diameter, protrusion and spring strength play a role too).

Do you care about how any of it feels? I don't think Mack Bros has released their trigger hangers yet but if or when they do release it, a different hanger moving your trigger rearward would theoretically increase your ignition reliability. If reliability is a concern, lean towards more firing pin fall. The benchrest crowd like around 0.240" to 0.250" (off the top of my head) firing pin fall for R700 clone style actions for best accuracy, reliable ignition, and good bolt lift/close feel.

 

[Halffull]

Indifferent (preference) as long as there's enough pin fall for reliable ignition (firing pin tip diameter, protrusion and spring strength play a role too).

Do you care about how any of it feels? I don't think Mack Bros has released their trigger hangers yet but if or when they do release it, a different hanger moving your trigger rearward would theoretically increase your ignition reliability. If reliability is a concern, lean towards more firing pin fall. The benchrest crowd like around 0.240" to 0.250" (off the top of my head) firing pin fall for R700 clone style actions for best accuracy, reliable ignition, and good bolt lift/close feel.

I'm probably not experienced enough to have a preference for feel. In fact, I think the afore mentioned R700 feels very nice with about .033 coc. There is a lot of effort made and information out there on reducing coc to near zero. However, there is little to no info on negative coc. I didn't know if this was a good problem or no problem at all.

The cock on open yields .2545 firing pin fall which as stated is near ideal. It seems wasteful to make the extra effort to cock that far and then reduce it on close. I'm sure the action will be reliable as is. It's just the idea of making it a little better.

I will say that this action seems like a lot of effort on open. Can I conclude that this is due to a heavy weight firing pin spring?

 

[Evintos]

I'm probably not experienced enough to have a preference for feel. In fact, I think the afore mentioned R700 feels very nice with about .033 coc. There is a lot of effort made and information out there on reducing coc to near zero. However, there is little to no info on negative coc. I didn't know if this was a good problem or no problem at all.

The cock on open yields .2545 firing pin fall which as stated is near ideal. It seems wasteful to make the extra effort to cock that far and then reduce it on close. I believe it's wasteful as well. I'm sure the action will be reliable as is. It's just the idea of making it a little better. Whenever Mack Bros releases the trigger hangers and if you can afford to, why not? There's no harm in getting near 0 movement for trigger timing. One thing to keep in mind is that different triggers have different sear positions so if you decide to get a TT trigger, your measurements will likely change.

I will say that this action seems like a lot of effort on open. Can I conclude that this is due to a heavy weight firing pin spring? It's a combo of the firing pin spring weight, cocking cam geometry & cocking piece geometry, potential bottom of cocking piece dragging on top of the trigger sear (due to the trigger sear being too tall), and even possible friction from the spring dragging on the inner diameter of the bolt body as it's being compressed. You can try and figure out which one it is slightly by adding a little oil (if you haven't done so already) on the metal on metal contact points for the underlined portions one at a time to see which one is causing the most drag.

 

[Halffull]

Thanks for the detailed explanation. The R700 trigger is temporary as my TT is currently mounted elsewhere, so any adjustments will be pending.

 

[4O6shootist]

I found my evo2 tactical with a tt special installed to have very little coc and 245k pinfall. Personally I'm going to pull my TT units, and start using bna tac sport triggers. I acquired my first a few months ago and I love it.

 

[Halffull]

Another observation/question I have. Honestly the bolt hangs/binds very easily when pushed forward from fully open. Any lateral force on the bolt when pushing forward causes it to stick. For example, pushing from the end of the bolt handle. Also, after installing in a stock, the mag follower pushing against the bottom of the bolt increases the drag dramatically, both opening and closing. The bolt is nitride treated per the manufacture's information. The bolt finish is smooth with regard to machining, but the coating/treatment feels almost tacky like new paint. Do I chalk this up to just being new and expect it will break in soon? Does it need a lot of oil to run smooth? This is not complaining. I have high expectations for "custom actions", and this is my first one.

 

[Average guy]

Another observation/question I have. Honestly the bolt hangs/binds very easily when pushed forward from fully open. Any lateral force on the bolt when pushing forward causes it to stick. For example, pushing from the end of the bolt handle. Also, after installing in a stock, the mag follower pushing against the bottom of the bolt increases the drag dramatically, both opening and closing. The bolt is nitride treated per the manufacture's information. The bolt finish is smooth with regard to machining, but the coating/treatment feels almost tacky like new paint. Do I chalk this up to just being new and expect it will break in soon? Does it need a lot of oil to run smooth? This is not complaining. I have high expectations for "custom actions", and this is my first one.

Shoot some Hornady dry lube on the bolt. That slicks stuff up. I’m sure it will wear in. My BIL has a Mack titanium action, loves it.

 

[4O6shootist]

Another observation/question I have. Honestly the bolt hangs/binds very easily when pushed forward from fully open. Any lateral force on the bolt when pushing forward causes it to stick. For example, pushing from the end of the bolt handle. Also, after installing in a stock, the mag follower pushing against the bottom of the bolt increases the drag dramatically, both opening and closing. The bolt is nitride treated per the manufacture's information. The bolt finish is smooth with regard to machining, but the coating/treatment feels almost tacky like new paint. Do I chalk this up to just being new and expect it will break in soon? Does it need a lot of oil to run smooth? This is not complaining. I have high expectations for "custom actions", and this is my first one.

Mine has a slight bind when full open and you intentionally side load it. But with proper manipulation its not bad. I'm tempted to have the action body and recoil lug dlc coated myself. It's definitely not as smooth as an impact or fuzion or nitride defiance. Mine doesn't sound as bad as yours does however. Its almost done, 27" 7.5tw MTU 6x47 lapua in a krg bravo chassis. Threw it together as a budget build for my brother's birthday. I do have another evo2 that'll prob become a 65cm target rig that's in the works. I've contemplated getting rid of it and getting another Archimedes too.

 

[Halffull]

Shoot some Hornady dry lube on the bolt. That slicks stuff up. I’m sure it will wear in. My BIL has a Mack titanium action, loves it.

Hornady One Shot or something else?

 

[Average guy]

Hornady One Shot or something else?

They make a dry lube also. Black can

 

[Evintos]

Another observation/question I have. Honestly the bolt hangs/binds very easily when pushed forward from fully open. Any lateral force on the bolt when pushing forward causes it to stick. For example, pushing from the end of the bolt handle. Also, after installing in a stock, the mag follower pushing against the bottom of the bolt increases the drag dramatically, both opening and closing. The bolt is nitride treated per the manufacture's information. The bolt finish is smooth with regard to machining, but the coating/treatment feels almost tacky like new paint. Do I chalk this up to just being new and expect it will break in soon? Does it need a lot of oil to run smooth? This is not complaining. I have high expectations for "custom actions", and this is my first one.

In general, I wouldn't worry about the follower drag unless it was causing cartridge feed issues. If there are feed issues, check to see if your stock has magazine latch height adjustment. Make sure to also thoroughly clean the bolt to remove any residue left over from the nitriding process (I like using non-chlorinated brake cleaner and paper towels), and then lubricate with a thin layer of low viscosity oil. The residue tends to be a light dusty brown to grey. A little Teflon non-stick dry film lubricant (PTFE spray) on the follower will also help with the friction a bit.

 

[Halffull]

In general, I wouldn't worry about the follower drag unless it was causing cartridge feed issues. If there are feed issues, check to see if your stock has magazine latch height adjustment. Make sure to also thoroughly clean the bolt to remove any residue left over from the nitriding process (I like using non-chlorinated brake cleaner and paper towels), and then lubricate with a thin layer of low viscosity oil. The residue tends to be a light dusty brown to grey. A little Teflon non-stick dry film lubricant (PTFE spray) on the follower will also help with the friction a bit.

This is a hinged floorplate so I assume there is no adjustment. I'll try the cleaning first and lube recommendations that have been mentioned.

Although I am slightly disappointed, this is not a dealbreaker for me in any way. I haven't had the luxury of handling any top name actions so therefore I can only compare to a factory R700. Maybe I'm being too picky. The R doesn't bind at all BTW.

 

[Evintos]

This is a hinged floorplate so I assume there is no adjustment. I'll try the cleaning first and lube recommendations that have been mentioned.

Although I am slightly disappointed, this is not a dealbreaker for me in any way. I haven't had the luxury of handling any top name actions so therefore I can only compare to a factory R700. Maybe I'm being too picky. The R doesn't bind at all BTW.

Just noticed it was a BDL. What stock and BDL bottom metal and box do you have? The internal box mag of the BDL could be too tall.

 

[4O6shootist]

This is a hinged floorplate so I assume there is no adjustment. I'll try the cleaning first and lube recommendations that have been mentioned.

Although I am slightly disappointed, this is not a dealbreaker for me in any way. I haven't had the luxury of handling any top name actions so therefore I can only compare to a factory R700. Maybe I'm being too picky. The R doesn't bind at all BTW.

Some night while you're watching a movie or listening to a podcast, something casual, just work the bolt open, closed, locked, repeat and it'll likely smoothen out. Firing a few hundred rounds will also likely help. My nucleus gen2 felt somewhat course rough and gritty, but after many dry fires and a few hundred rounds it felt a lot better. It has closer to 2k on it now, and it runs like a sewing machine. Actions do take a lil time to smoothen out and break in.

 

[Halffull]

Just noticed it was a BDL. What stock and BDL bottom metal and box do you have? The internal box mag of the BDL could be too tall.

This is a Boyd's stock and Rem bottom metal. I was just looking closely at that. The rear of the box mag is close but it has clearance. The box is not long enough though. That is with the box sitting all the way forward there is a gap at the back between the box and the action. That is because the action is made to accept longer box mags right?

 

[Halffull]

Upon closer examination the bolt hangs the worst when it is all the way open with an upward bias on the bolt pushing forward. This is with the bolt head completely in the rear of the receiver. Once it moves in an inch or so it travel much better. This must create the steepest angle and it binds hard. Pressure to the right binds some and left pressure not so much. A downward bias on the bolt moves it forward with ease and is quite smooth. I think the pinch point is the top of the bolt and the inside of the top of the receiver at the very back where the bolt handle turns. This is a fairly sharp 90* cut. Seems like a very light chamfer at this location would help. As mentioned, I'm hoping time will even it out.

 

[Evintos]

This is a Boyd's stock and Rem bottom metal. I was just looking closely at that. The rear of the box mag is close but it has clearance. The box is not long enough though. That is with the box sitting all the way forward there is a gap at the back between the box and the action. That is because the action is made to accept longer box mags right?

Yeah it's cut for 2.990" cartridge lengths. You'd likely need something like the Wyatt's MBE-05 kit (depends on which caliber you're shooting) which the has a longer follower and longer inside/outside length internal box if you want to stick with internal box mag. Also likely gonna need new BDL bottom metal to accommodate the increase in internal box mag length.

 

[Halffull]

As for you all that have built a rifle using these actions, did you check to see if the action and recoil lug was square? Did you need to lap the lugs? I would assume tolerances are very good and wouldn't need any trueing.

 

[Halffull]

Mine has a slight bind when full open and you intentionally side load it. But with proper manipulation its not bad. I'm tempted to have the action body and recoil lug dlc coated myself. It's definitely not as smooth as an impact or fuzion or nitride defiance. Mine doesn't sound as bad as yours does however. Its almost done, 27" 7.5tw MTU 6x47 lapua in a krg bravo chassis. Threw it together as a budget build for my brother's birthday. I do have another evo2 that'll prob become a 65cm target rig that's in the works. I've contemplated getting rid of it and getting another Archimedes too.

Did you find the recoil lug to be square with barrel shoulder? Did you have to lap the bolt lugs?

 

[4O6shootist]

Did you find the recoil lug to be square with barrel shoulder? Did you have to lap the bolt lugs?

My smith said when trying to measure headspace dimensions from lug to bolt face the dimensions weren't square, we chalked that up the tight funky fit of the lugs hex shape not being fully seated somewhere. We tapped it down pretty good with a mallet, but never got perfect numbers. The tenon print/headapace dimensions shipped with action didn't match what he measured and cut. Personally, I'd probably spring the money for a nuc gen2, kelbly atlas next time. I like the origin for what it is, but having to mill the action to run 2.95 internal aics mags or extended wyatts mags, should be included with the action, just re-write the cnc program.

 

[Halffull]

My smith said when trying to measure headspace dimensions from lug to bolt face the dimensions weren't square, we chalked that up the tight funky fit of the lugs hex shape not being fully seated somewhere. We tapped it down pretty good with a mallet, but never got perfect numbers. The tenon print/headapace dimensions shipped with action didn't match what he measured and cut. Personally, I'd probably spring the money for a nuc gen2, kelbly atlas next time. I like the origin for what it is, but having to mill the action to run 2.95 internal aics mags or extended wyatts mags, should be included with the action, just re-write the cnc program.

I'm a little concerned with the recoil lug design. Although the indexed lug design is innovative, I am skeptical of how it can achieve tight tolerances. It seems like the lug/action interface contacts on the twelve point index rather than on the flat faces of the action and lug. This appears difficult to measure as you stated and moreover difficult to fix if it isn't true. Worst scenario is to mill off the indexed portion on the receiver face and go to a common flat recoil lug.

Anyway, this is purely speculative on my part as I haven't yet quantified any problems. I don't want to run anyone's product down based on my assumptions. If we (my smith) find any issues I will contact MB and look for resolutions. Otherwise I need to give them the benefit of the doubt.

 

[Barelstroker]

You should have joined "The Hide" before you committed to the action.
I viewed the website & they gave me a choice of "Standard action length" or nothing.
What is standard?
You may be able to make a go of this action or not but, I think you'd have done yourself a greater justice by asking 1st, buying 2nd.

 

[4O6shootist]

I'm a little concerned with the recoil lug design. Although the indexed lug design is innovative, I am skeptical of how it can achieve tight tolerances. It seems like the lug/action interface contacts on the twelve point index rather than on the flat faces of the action and lug. This appears difficult to measure as you stated and moreover difficult to fix if it isn't true. Worst scenario is to mill off the indexed portion on the receiver face and go to a common flat recoil lug.

Anyway, this is purely speculative on my part as I haven't yet quantified any problems. I don't want to run anyone's product down based on my assumptions. If we (my smith) find any issues I will contact MB and look for resolutions. Otherwise I need to give them the benefit of the doubt.

My lugs on both bolts after a lot of dry fire work appear to have consistent wear on both sides indicating good contact to the abutments.

 

[b6graham]

My smith said when trying to measure headspace dimensions from lug to bolt face the dimensions weren't square, we chalked that up the tight funky fit of the lugs hex shape not being fully seated somewhere. We tapped it down pretty good with a mallet, but never got perfect numbers. The tenon print/headapace dimensions shipped with action didn't match what he measured and cut. Personally, I'd probably spring the money for a nuc gen2, kelbly atlas next time. I like the origin for what it is, but having to mill the action to run 2.95 internal aics mags or extended wyatts mags, should be included with the action, just re-write the cnc program.

i run 2.955 coal cartridges in AICS mags no problem with origins. and the newest generation can go even longer

 

[Halffull]

My lugs on both bolts after a lot of dry fire work appear to have consistent wear on both sides indicating good contact to the abutments.

I would say the same here. I've cycled the bolt enough to develop wear patterns, and since there is not a barrel attached, I can look at the bearing surface of the abutments. All surfaces seem to have the consistent patterns as does yours. I will say that the abutments are shinier on the very outside (OD) edge and inside (ID) edge. In other words, the middle of the abutment isn't showing wear yet. There must be a slight machining ridge in these areas and can't be much. However, both abutments look the same and that I think is most important. I think this will wear in soon.

In spite of my nitpicking, I do like this action so far. I would probably have done the same with any other brand. It will have to pass the squareness test at the recoil lug/barrel shoulder junction though. It may be a week or two before I find out. I will update with results.

 

[Halffull]

You should have joined "The Hide" before you committed to the action.
I viewed the website & they gave me a choice of "Standard action length" or nothing.
What is standard?
You may be able to make a go of this action or not but, I think you'd have done yourself a greater justice by asking 1st, buying 2nd.

I have the Evo II stainless short model. I see them offered as either short or long options. I think the "standard" is for the bolt face option, which appears to be the only option.

 

[4O6shootist]

I would say the same here. I've cycled the bolt enough to develop wear patterns, and since there is not a barrel attached, I can look at the bearing surface of the abutments. All surfaces seem to have the consistent patterns as does yours. I will say that the abutments are shinier on the very outside (OD) edge and inside (ID) edge. In other words, the middle of the abutment isn't showing wear yet. There must be a slight machining ridge in these areas and can't be much. However, both abutments look the same and that I think is most important. I think this will wear in soon.

In spite of my nitpicking, I do like this action so far. I would probably have done the same with any other brand. It will have to pass the squareness test at the recoil lug/barrel shoulder junction though. It may be a week or two before I find out. I will update with results.

Talked to my smith again this afternoon, after cutting tenon dialed into 0001 of bore/grooves, he torqued the action to the tenon, said it was square.