Gunsmithing Shorten lead or throat length?

[Thumper580]

I have a 24" bull barrel for my 308 gas gun. The present bullet jump is too much (.100). Can the barrel extension be removed, retreaded and extension reinstalled? Or simpler to cut off the extension section and install new extension with shorter bullet jump (.010"). Are both doable or not. Suggestions on smith to do the work. Thanks.

 

[FCS]

Cutting off extension, the treads on barrel that extension threads onto, is going to mess with your port location. You'll need a custom gas tube if you move chamber forward by 0.0625 (1 thread as threads are 16 TPI) or 0.125 (two threads). You'll also have to time port and pin on bbl extension so they are both TDC. Not worth the hassle, if jump is too much then sell off this barrel and get barrel that has "correct" chamber.

You have 0.100" jump or 0.100" freebore dimension?

Most OTM bullets are not going to be bothered at all by 0.100" jump. I jump VLDs 0.040, IMHO you are just asking for issues to try running ammo with 0.010" jump.

YMMV

 

[Thumper580]

Cutting off extension, the treads on barrel that extension threads onto, is going to mess with your port location. You'll need a custom gas tube if you move chamber forward by 0.0625 (1 thread as threads are 16 TPI) or 0.125 (two threads). You'll also have to time port and pin on bbl extension so they are both TDC. Not worth the hassle, if jump is too much then sell off this barrel and get barrel that has "correct" chamber.

You have 0.100" jump or 0.100" freebore dimension?

Most OTM bullets are not going to be bothered at all by 0.100" jump. I jump VLDs 0.040, IMHO you are just asking for issues to try running ammo with 0.010" jump.

YMMV

When seated in the mag the bullets are jumping about 100 thousandths. If I set the bullet depth to be .010 off the lands and handfeed in the gun it shoots incredible groups. To achieve this I guess I need to get another barrel, provide them with a loaded dummy round so they can make sure the lands are about .010 away.

 

[chipsfan]

When seated in the mag the bullets are jumping about 100 thousandths. If I set the bullet depth to be .010 off the lands and handfeed in the gun it shoots incredible groups. To achieve this I guess I need to get another barrel, provide them with a loaded dummy round so they can make sure the lands are about .010 away.

Did you try loading some rounds at your desired length and feeding them one at a time- kinda like the XC guys do with 80 SMKs at the 600 yard line and see how it works out?

 

[Thumper580]

Did you try loading some rounds at your desired length and feeding them one at a time- kinda like the XC guys do with 80 SMKs at the 600 yard line and see how it works out?

That's what I was doing... .010 off the lands and slipping the round in the chamber then dropping the bolt. Doing that it shoots wonderful (1/2"). Was trying to get the same results with loading in the mag(2.800 OAL) but groups open up a lot. I'm after accuracy so I guess I can live with single loading vs $500 for a custom cut barrel.

 

[FCS]

That's what I was doing... .010 off the lands and slipping the round in the chamber then dropping the bolt. Doing that it shoots wonderful (1/2"). Was trying to get the same results with loading in the mag(2.800 OAL) but groups open up a lot. I'm after accuracy so I guess I can live with single loading vs $500 for a custom cut barrel.

168 SMK or 175 SMK will take that much jump w/o issue. Should easily get you below 3/4 MOA regularly and 0.5 MOA isn't unreasonable.
What is your load?
What barrel?

 

[chipsfan]

That's what I was doing... .010 off the lands and slipping the round in the chamber then dropping the bolt. Doing that it shoots wonderful (1/2"). Was trying to get the same results with loading in the mag(2.800 OAL) but groups open up a lot. I'm after accuracy so I guess I can live with single loading vs $500 for a custom cut barrel.

How does it shoot with the .100" jump?

 

[chipsfan]

168 SMK or 175 SMK will take that much jump w/o issue. Should easily get you below 3/4 MOA regularly and 0.5 MOA isn't unreasonable.

This is what I am thinking

 

[Unknown]

I had a couple of 6.5 Grendels and there was about 7 thousandths difference between the headspace length on the two rifles. I didn't want to work my brass that hard, nor did I want to have two different loads of ammo around. So I contacted Harrison at AR15performance, and sent the rifles off to have the headspace made closer between the two rifles. The rifles came back with 1.5 thousandths difference between the two headspaces, and I can live with that. Both rifles shoot great, and my brass is living a long time. I quit counting when my reloads went over 6 for the cases. If I don't anneal the necks split, that that is no surprise, so I anneal.

Because the Grendel's neck dimension is 5 thousandths larger than the 264 LBC reamer that was used on my rifles, Harrison did tell me that there would be a few thousandths difference (smaller) at the end of the neck because both rifles had a Grendel chamber and throat. As far as I can see, the small difference doesn't effect anything negatively.

Apparently, there can be some very small differences between the sizes of the various barrel extensions, and by adding a larger one to one rifle and a smaller one onto the other, things came out great. I'm not sure if I really understood how he worked things out to even out the headspace, so forgive me if my attempt at explanation isn't very good.

 

[Thumper580]

How does it shoot with the .100" jump?

Poor by my standards.... 1" to 1-1/2"

 

[FCS]

Poor by my standards.... 1" to 1-1/2"

Any difference between the load that is 0.100" jump and 0.010" jump other then OAL?
Running same powder charge?

 

[Thumper580]

Any difference between the load that is 0.100" jump and 0.010" jump other then OAL?
Running same powder charge?

Everything is the same... .100" jump 1" to 1-1/2" group. .010" jump 1/2" groups

 

[chipsfan]

Everything is the same... .100" jump 1" to 1-1/2" group. .010" jump 1/2" groups

If you single feed the .100" jump round how does it shoot? I'm wondering if how this bad boy is locking up is affecting your group size.

 

[Thumper580]

If you single feed the .100" jump round how does it shoot? I'm wondering if how this bad boy is locking up is affecting your group size.

They produce the larger groups... Convinced it's the extreme jump that's messing it up.

 

[FCS]

Everything is the same... .100" jump 1" to 1-1/2" group. .010" jump 1/2" groups

You need to adjust your powder charge. Adjust load and your groups with 0.100" jump will tighten up.

What is load?

 

[Pusher591]

Just curious and not flaming the OP, I generally would like to know if there is a reason for single loading a gas gun or sticking with a gas gun that only shoots accurately if fed single loads.

 

[Thumper580]

You need to adjust your powder charge. Adjust load and your groups with 0.100" jump will tighten up.

What is load?

168 match with 4064 from 39 - 44 grs and 4895 from 39 - 43grs. No where in the range did it get better when set at 2.80 OAL and fed from the magazine.

 

[FCS]

168 match with 4064 from 39 - 44 grs and 4895 from 39 - 43grs. No where in the range did it get better when set at 2.80 OAL and fed from the magazine.

Any pressure signs? If not, keep going up with 4064. Better yet, switch to 175 SMKs and Varget.,
Which 168 match bullet? SMK, Berger, Hornaday, Speer, or ????
What blank and who chambered bbl?
Port size?
Port location?
Adjustable gas block?
Running a standard stock / buffer or stock that uses CAR buffer?