POI Shift - MDT Chassis

[aa909]

Trying to trouble shoot a new rifle build and hoping the good folks here can give me some useful feedback.

Build:
TS Custom barreled action: 28 Nosler
Lone Peak Fuzion Action
Benchmark barrel: 29" MTU
MDT ACC Chassis - Long Action
Fat Bastard brake

Ammo: 190gr Berbger LR Hybrid Target (Custom reloads Dallas)

for background, TS Custom built the barrel action and I purchased the MDT Chassis directly from MDT and dropped in the barreled action.

The issue I'm having is the POI will randomly shift 1"-2" at 100 yards from shot to shot. The pictures below is representative of what's happening, you see the first 2 rounds next to each other (bottom 2), followed by the next two rounds up about 1.5".

Same thing happens on nearly every target, like the one below: first 1 down, then 1 up, 1 down, 1 up. The POI shift seems to be in a 2 MOA range, and it has been consistently happening over the first 100 rounds fired.

I checked the action screws and they were tight at 60 lbs, the muzzle brake was not loose and the scope mount & rings were also tight. But if everything is tight how can the POI shift so much between shots?

I also spoke with Travis at TS Customs and Chad at Custom Reloads Dallas and they both said the chassis may need bedding. I thought bedding a chassis may prevent POI shifting over time, but would it help with what's happening here where the POI is shifting 2 MOA from shot to shot?

Look forward to everyone's feedback in terms of what the problem may be and any troubleshooting ideas you have are greatly appreciated.

Thanks
Art

 

[HMRamateur]

Tagging in for info... can't answer your question, but have you spoken to MDT about it? They claim their chassis doesn't need bedding, curious what they would have to say about that.

 

[buglum]

I would be interested in this, I have the same exact scenario happening in the new Manners LRH with mini chassis. Manners told me to re torque the action screws, they were tight. I really don’t want to bed it, that defeats one of the purposes of a chassis in my opinion.

 

[just browsing]

With rifle on a bipod and both action screws torqued to spec, loosen the front one (leave the rear tight). If the action moves or “lifts up” in the chassis, there is tension on the receiver and it should be bedded.

Fwiw I’ve seen this happen with a number of MDT chassis - ACC, HNT, LSS, and XRS.

 

[aa909]

With rifle on a bipod and both action screws torqued to spec, loosen the front one (leave the rear tight). If the action moves or “lifts up” in the chassis, there is tension on the receiver and it should be bedded.

Fwiw I’ve seen this happen with a number of MDT chassis - ACC, HNT, LSS, and XRS.

this evening I separated the chassis to inspect for any obvious issues, but after reading your post I reinstalled it and tried your suggestion. the barrel didn’t lift up when I loosened the front action screw, But it is possible the original install may have had tension that didn’t repeat with the reinstall.

Tagging in for info... can't answer your question, but have you spoken to MDT about it? They claim their chassis doesn't need bedding, curious what they would have to say about that.

I’m calling them tomorrow morning. I’ll report back when I get their feedback.

I would be interested in this, I have the same exact scenario happening in the new Manners LRH with mini chassis. Manners told me to re torque the action screws, they were tight. I really don’t want to bed it, that defeats one of the purposes of a chassis in my opinion.

Agree, I don‘t want to bed mine either, it defeats the purpose of having a chassis system.

 

[Gee Kay]

rear bag or something else?

i also find it easier to aim at a target that isnt round.

 

[kcormack]

Riflescope?

 

[Terry Cross]

Just for shits, swap another scope onto the rifle and repeat the group attempts.
Very quick and easy to eliminate a variable.

./

 

[ShtrRdy]

If a different scope doesn't help I'm thinking the load doesn't work well in that rifle. Try some different ammunition to see if that makes a dfference.

 

[Dave62677]

My friend have the same issue on his mdt long action as well. We shot handload, factory load, swap scope, torque check on scope ring, scope mount, action screw, swap muzzle brake, re torque the barrel. Loading heavy, mild light pressure to bipod, Nothing work to eliminate the vertical spread. The barrel sent back to proof and proof shot the barrel and produces .5 moa on factory load.

 

[Emerson0311]

With rifle on a bipod and both action screws torqued to spec, loosen the front one (leave the rear tight). If the action moves or “lifts up” in the chassis, there is tension on the receiver and it should be bedded.

Fwiw I’ve seen this happen with a number of MDT chassis - ACC, HNT, LSS, and XRS.

Very interesting. I’ve got an Oryx with a Howa 300 PRC barrelled action in it that has some weird poi shifts I am having trouble tracking down. Already switched optics and checked rings, rail, action screw tightness. 100 rnds downrange, factory and reloads and no solution yet. Bedding is something cheap to try.

 

[aa909]

If a different scope doesn't help I'm thinking the load doesn't work well in that rifle. Try some different ammunition to see if that makes a dfference.

I tried Nosler 150gr and custom Berger 184gr and 190gr LR Hybrids, all 3 had similar POI shifts.

Just for shits, swap another scope onto the rifle and repeat the group attempts.
Very quick and easy to eliminate a variable.

./

Yes that's going to be part of the plan. I'm waiting for MDT to get back to me with their feedback before I take the rifle back out for more testing but the plan is as follows:

Reinstall the barreled action and chassis to make sure there's no obvious tension.
Remove muzzle brake
Swap scopes (I've got an identical ATACR I'll swap from my 6.5 PRC)
Ammo: Berger 180gr, 190gr & 195gr LR Hybrid match (LOL trouble shooting 28 Nosler given today's ammo prices is Sofa King painful)

I'll report back after this next round of trouble shooting but if I'm still getting sloppy POI shifts then it's most likely that the chassis needs to be bedded which I don't want to do. I have other chassis rifles (MPA 6.5 PRC, MPA 300 PRC, TikKa 6.5 Creedmoor) which shoot 0.5 MOA and don't need to be bedded.

Stay tuned for an update, but in the mean time please keep chiming in with your thoughts and guidance. I'll try to incorporate your feedback into the next round of testing.

- Art

 

[6brshooter]

I've saw several mdt chassis that needed rear tang bedded. There was a fulcrum created by inlet, right in front of rear screw. Generally when you checked for action stress, and front screw was loosened, barrel would move up 3/16+". Generally better to test with barrel pointing up, maybe 80°. A long, heavy barrel might mask any movement if it's on the horizontal plane during test.

Thats a lot of movement if caused by bedding, I'm betting optic or mount. Might pull brake off and test without it.

 

[Hawk in WY]

I see that vertical pattern with an unstable front rest or inconsistent recoil management in particular with magnum calibers.

The bullet is still in the bore when the rifle recoils. Some rifles like to free recoil straight back. Some like to be held down.

Try shooting from a solid front rest and pay particular attention to what the rifle is doing under recoil. Try different holds.

Good luck.

 

[kthomas]

I see that vertical pattern with an unstable front rest or inconsistent recoil management in particular with magnum calibers.

The bullet is still in the bore when the rifle recoils. Some rifles like to free recoil straight back. Some like to be held down.

Try shooting from a solid front rest and pay particular attention to what the rifle is doing under recoil. Try different holds.

Good luck.

Could also be improper rear bag management. Larger recoiling rifles will certainly exploit any inconsistencies in our application of the fundamentals.

 

[rijndael]

IME, groups that look like doubles is usually the charge weight being in between nodes.

 

[kthomas]

IME, groups that look like doubles is usually the charge weight being in between nodes.

I'm going to respectfully disagree.

A rifle that's "between nodes" is not going to have that large of discrepancy in POI shifts at 100 yards.

 

[rijndael]

I'm going to respectfully disagree.

A rifle that's "between nodes" is not going to have that large of discrepancy in POI shifts at 100 yards.

I just went to the basement to see if I saved my targets from the past couple of weeks, unfortunately I didn't, but I'm going to keep looking. I had a 6XC doing doubles. I just changed jugs of powder, from the same lot (so I didn't think anything of it), and it started doubling (.75" to 1" apart). I was certain that my NF scope or something was askew and went down the rabbit hole of swapping scopes, checking base screws, and re-testing. I ended up dropping almost 1 grain of powder off the charge and it's back to a .25" group @ 100.

 

[Baron23]

I tried Nosler 150gr and custom Berger 184gr and 190gr LR Hybrids, all 3 had similar POI shifts.


Yes that's going to be part of the plan. I'm waiting for MDT to get back to me with their feedback before I take the rifle back out for more testing but the plan is as follows:

Reinstall the barreled action and chassis to make sure there's no obvious tension.
Remove muzzle brake
Swap scopes (I've got an identical ATACR I'll swap from my 6.5 PRC)
Ammo: Berger 180gr, 190gr & 195gr LR Hybrid match (LOL trouble shooting 28 Nosler given today's ammo prices is Sofa King painful)

I'll report back after this next round of trouble shooting but if I'm still getting sloppy POI shifts then it's most likely that the chassis needs to be bedded which I don't want to do. I have other chassis rifles (MPA 6.5 PRC, MPA 300 PRC, TikKa 6.5 Creedmoor) which shoot 0.5 MOA and don't need to be bedded.

Stay tuned for an update, but in the mean time please keep chiming in with your thoughts and guidance. I'll try to incorporate your feedback into the next round of testing.

- Art

Ah, Art....perhaps a suggestion...just change one variable at a time. Otherwise, you may know you fixed something but not what that something was, exactly. Right...?

E.g. - reinstall in the chassis...shoot and see what you got. Then, remove brake, shoot, see what you got. THEN swap scopes...or change the order but keep your fault isolation by changing one at a time and then testing.

Just a suggestion.

 

[Feniks Technologies]

this evening I separated the chassis to inspect for any obvious issues, but after reading your post I reinstalled it and tried your suggestion. the barrel didn’t lift up when I loosened the front action screw, But it is possible the original install may have had tension that didn’t repeat with the reinstall.



I’m calling them tomorrow morning. I’ll report back when I get their feedback.



Agree, I don‘t want to bed mine either, it defeats the purpose of having a chassis system.

The whole “shouldn’t bed a chassis” or “defeats purpose” is a myth. And has been perpetuated so much online that it’s become an unrealistic expectation.

When you mass product a chassis for “universal” fit, it’s impossible that it will mate perfectly with every individual action produced.

 

[srt-4_uk]

I'm not sure why anyone would think MDT chassis don't need bedded


On a completely unrelated note

 

[Emerson0311]

I'm not sure why anyone would think MDT chassis don't need bedded


On a completely unrelated note

Nice presentation from Ryan.

 

[Feniks Technologies]

I'm not sure why anyone would think MDT chassis don't need bedded


On a completely unrelated note

Couple points:

- They said it’s an exact fit for the action it’s inlet for. So, one would need to know which custom action they modeled the 700 drop in. Was it an actual 700 or one of the customs? The OP has a lone peak which isn’t completely round. I can almost guarantee they didn’t use that action as a model.

- Companies are typically speaking generally when that talk about things like this. There’s always exceptions.

But I agree with your point, information like this can confuse consumers.

 

[6brshooter]

IME, groups that look like doubles is usually the charge weight being in between nodes.

2&3 groups are usually a result of bedding issue or barrel contact issue.

 

[Deleted member 115591]

AA909, was wondering if you figured out any more information on the POI?

I am having a similar issue with really bad groups for a 300prc in a MDT Chassis with a Proof prefit.

I reached out to Proof and after sending the barrel back to them, they verified that the issue is not the barrel.

Which leads me to believe that the chassis might be the cause. I reached out to MDT but they responded by saying their chassis does not need to be bedded.

 

[Emerson0311]

AA909, was wondering if you figured out any more information on the POI?

I am having a similar issue with really bad groups for a 300prc in a MDT Chassis with a Proof prefit.

I reached out to Proof and after sending the barrel back to them, they verified that the issue is not the barrel.

Which leads me to believe that the chassis might be the cause. I reached out to MDT but they responded by saying their chassis does not need to be bedded.

I’ve got a factory barreled Howa 300PRC in an Oryx that seems similar. I will bed it, but because I’ve never a bedding job, I’ll put it off until I have a block of time and some angst to work through.

 

[Mike Casselton]

I see that vertical pattern with an unstable front rest or inconsistent recoil management in particular with magnum calibers.

The bullet is still in the bore when the rifle recoils. Some rifles like to free recoil straight back. Some like to be held down.

Try shooting from a solid front rest and pay particular attention to what the rifle is doing under recoil. Try different holds.

Good luck.

^^^^
Totally agree it's caused by the bag or you're rolling your shoulder forward.
If the patterns are consistently verticle it's in the way you're driving the rifle.

 

[DairyDictator]

I noticed this happening when running my bipod all the way at the end of the arca rail on a heavy rifle. I moved by bipod back a few inches and it cleared up. I'm not convinced the gen 1 forends are quite strong enough.

 

[kindred_spirits]

I'm interested if you figured anything out as well. I recently got a barreled action built with a Lone Peak Fuzion S/A and when I was at the gunsmith getting it assembled in the MDT ACC they said that the action was rocking back and forth with either of the action screws loosened. They mentioned that they've seen this issue before (with various actions, Defiance, Lone Peak, Impact) and sent the chassis back to MDT, who replaced the chassis. They didn't have the same issue with the replacements.

I haven't shot mine yet (still waiting on scope rings) so I don't know if I'll experience the same issue as you, but I thought this might be useful.

 

[WB300]

Tag...

 

[Deleted member 115591]

I'm interested if you figured anything out as well. I recently got a barreled action built with a Lone Peak Fuzion S/A and when I was at the gunsmith getting it assembled in the MDT ACC they said that the action was rocking back and forth with either of the action screws loosened. They mentioned that they've seen this issue before (with various actions, Defiance, Lone Peak, Impact) and sent the chassis back to MDT, who replaced the chassis. They didn't have the same issue with the replacements.

I haven't shot mine yet (still waiting on scope rings) so I don't know if I'll experience the same issue as you, but I thought this might be useful.

Yeah, when I spoke to Proof after they tested the barrel, they hinted at the issue being the chassis. They suggested that I bed the chassis to the action.

 

[Bo Ployhar]

If it were just double grouping randomly, I would lean towards load in-between nodes. But considering it is consistently vertical my guess is improper bag/bipod set up and or inconsistent form/shooting. For that little of movement it takes a very slight difference in shooting, slight enough that you might swear it isn't you. If you are not already running a vertical grip on the chassis that is the first thing I would do. The angled grips make it almost impossible to to do everything exactly the same every time.
Also with any larger guns I get way better consistency by "driving them", some forward pressure and then pull it into you firmly. It feels and looks worse in the scope but the results are way better.

 

[aa909]

Alright, finally had a chance to go back out to the range this morning . Let me first say thank you for everyone's feedback, it's much appreciated and I incorporated your feedback in my session today (e.g. focusing on rear bag control, loading the bipod, grip pressure etc).

Recap of troubleshooting:
- Removed and reinstalled chassis ensuring proper torque sequence and specs.
- Reinstalled scope and confirmed torque specs on the mount / rings.
- Removed muzzle brake (reinstalled at range after shooting a few groups w/o brake).
- Ammo, Berger 184gr, 190gr & 195gr Hybrid match ammo

Results below are after a couple of cold bore shots, followed by 5 rounds without the brake to ensure I'm not getting any fliers.
Here is the first 3-shot group with everything reinstalled including muzzle brake (190gr Berger):

As you can see the POI is waaaaaaaaaaaaaay off from the original zero but the group is tight (0.5 MOA @ 100 yards).

This is the first 3 shot group after zeroing the scope again (0.5 MOA at 100 Yards): 195gr Berger

As you can see in the first picture the POI was off vs where it was originally zero'd. In my experience the POI shouldn't have moved by 2.5" up and 2.5" right from simply reinstalling the chassis and reinstalling the scope mount (I never removed the scope from the mount, I only removed the mount from the action and reinstalled). I can only assume There must have been some tension in the chassis from the original install, but appreciate hearing your thoughts.

After zeroing the scope I went out to 600 yards today and was hitting 1 Moa targets pretty consistently. I definitely had a few shanked shots, however I would attribute those to human error, especially since the groups at 100 yards were tight.

Let me know what you guys think, and thanks again for everyone's comments.

- Art

 

[Alabusa]

I had that happening on my Sako M995 build and it turned out to be the rear ring was loose.

 

[Kohl604]

It’s an MDT , they’re not exactly stress free.
Get your front action screw hand tight , then tighten and loosen the rear action screw.
Place your hand on the barrel and front of the forend and feel for movement as you tighten and loosen.
If movement is detected it definitely needs to be bedded.
If no movement is immediately detected with previous test take and have your local gunsmith place an indicator on the barrel with the stock in a vise.
.001”-.004” is acceptable.
Then you can rule that out.

Best of luck!

 

[Oldloser]

Parallax? I know I have to do a perfect parallax at the zero line before I get going. This takes a few minutes with my scope.

Is your scope base really a perfect fit on the pic rail? I had some factory vortex rings that were not perfect and would shift a bit after a few rounds.

Was your scope really tight in the rings? I have had a POI shift when my scope's bolts loosened after a few trips from weather in teens to a warm car/house.

Finally was your action and chassis screws set right? You should tighten both screws then stand the rifle up, loosen, then tighten top screw then bottom.

My MDT ACC - Impact Action is bedded. I have a spuhr mount. With this I can take it apart all the way down to action, barrel, trigger, scope/mount, and when I reassemble (and check parallax) and then its .1-.2 mil off. My poi never shifts more than that.

 

[carbonbased]

You ever figure this out? (As in, did the groups stay small?)

Recoilweb had a good rundown of a bunch of chassis. Of the tested ones, all had v-block bedding except KRG, which had radius bedding. The latter is theoretically superior, according to Recoilweb, FWIW.

Some chassis experienced tang dive, and the ACC was one of them.

Best Long Range Chassis: Precision & Accuracy

From heavy PRS chassis to lightweight hunting chassis, we've tested dozens! What chassis is right for you?

www.recoilweb.com

 

[Baron23]

Alright, finally had a chance to go back out to the range this morning . Let me first say thank you for everyone's feedback, it's much appreciated and I incorporated your feedback in my session today (e.g. focusing on rear bag control, loading the bipod, grip pressure etc).

Recap of troubleshooting:
- Removed and reinstalled chassis ensuring proper torque sequence and specs.
- Reinstalled scope and confirmed torque specs on the mount / rings.
- Removed muzzle brake (reinstalled at range after shooting a few groups w/o brake).
- Ammo, Berger 184gr, 190gr & 195gr Hybrid match ammo

Results below are after a couple of cold bore shots, followed by 5 rounds without the brake to ensure I'm not getting any fliers.
Here is the first 3-shot group with everything reinstalled including muzzle brake (190gr Berger):
View attachment 7809074

As you can see the POI is waaaaaaaaaaaaaay off from the original zero but the group is tight (0.5 MOA @ 100 yards).

This is the first 3 shot group after zeroing the scope again (0.5 MOA at 100 Yards): 195gr Berger
View attachment 7809078


As you can see in the first picture the POI was off vs where it was originally zero'd. In my experience the POI shouldn't have moved by 2.5" up and 2.5" right from simply reinstalling the chassis and reinstalling the scope mount (I never removed the scope from the mount, I only removed the mount from the action and reinstalled). I can only assume There must have been some tension in the chassis from the original install, but appreciate hearing your thoughts.

After zeroing the scope I went out to 600 yards today and was hitting 1 Moa targets pretty consistently. I definitely had a few shanked shots, however I would attribute those to human error, especially since the groups at 100 yards were tight.

Let me know what you guys think, and thanks again for everyone's comments.

- Art

Well, Art…I guess you didn’t like that idea of only changing on thing at a time to preserve fault isolation.

So, now it’s improved but you can know exactly why.

Something to keep in mind for the future. It’s a basic concept in testing.