AR10 Neck tension Issues

[Teflon]

The only way I can see the bullet moving forward to due to the abrupt change in velocity when the round is chambered. The bullet shifting forward can only happen if there is a force on the bullet in the direction of motion. A rapid change in velocity of the brass the friction between the neck of the brass and bullet may not be able to apply a large enough force on the bullet to keep it from sliding forward in the casing.

To visualize what I am talking about think about inertial bullet pullers, the bullet comes out because the motion is abruptly changed. The heavier piece in the puzzle does not have a large enough force (the friction between the neck and the bullet) on it to keep it in the casing.

I have done enough physics and shooting/gunsmithing on ARs that I believe I have described the only mechanism that will cause the bullet to shift forward in the case, assuming that it is not a heavily compressed load. With that in mind there are a number of ways this can/may be remedied.

Either the force between the bullet and the casing needs to be increased with increase friction between case neck and bullet, by one or more of the following:

1. By reducing carbon on interior of case necks carbon acts to reduce friction,
2. reducing the neck diameter further when sizing by reducing ID of bushing (assuming you are using bushing dies) or
3. mechanical interference via crimping.

Alternatively or in combination, reducing the forward velocity of the bolt carrier group (BCG) which you might be able to achieve by a heavier BCG, heavier buffer, and/or lighter spring. The forward velocity of the BCG is strictly a function of the spring strength (variable), distance spring is compressed (fixed) and the total weight of the buffer and BCG (variable). These changes are just trying to keep the bolt from slamming forward too hard.

I find that a lot of AR=10's are over gassed and they can be helped by a heavier buffer so this is something you might try as it is sometimes easier on the brass. Too heavy is not good as it can mess with timing like all else there is a happy medium and sometimes it can actually be found. Operating too violently and an adjustable gas block is a good idea to throttle down impulse at the beginning of the process.

Newton is whispering in my ear (a mental malady of many people with too much physics work) and says try a lighter bullet as it will require less force to keep it in position in the case. Unfortunately, the lighter bullet may not do what you want to do.

Hope this helps,
wade

First i want to say thank you.
I’m using brand new Lapua brass. I do have adjustable gas block and i will adjust that as you mentioned in your post. I will shoot for accuracy and ES with .003 tension and see how it performs. If results are shit i will increase neck tension. If that is not working i will adjust gas block. If that is not working i will focus on BCG and spring system. But i will do one change at the time and test and i will update here info i get.

Im just amazed how much help im getting from you guys and i want to say thank you. I do have family and lil baby so it will take me lil bit i need time. But i will keep you updated as i test I will update results here.

 

[Teflon]

All of my semi auto rifles, M1a, Mini 14, Mini 30, AR 15 and AR 10"s have a tendency for the bullets to creep forward when sling shoting the bolts with loaded magazines. As long as it's less than .005", I don't worry about it. I sonic clean brass after sizing which leaves the inside of the inside case necks really "grippy", which helps hold the bullets tightly. I have tried crimping but it requires a surprisingly aggressive crimp to stop the creeping completely on the non-canelured bullets I like. That heavy-ish crimp compromises accuracy some in my experience so I don't crimp.

Robert thank you for your feedback. I also wash my brass. Jump im experiencing is .002 on average. Sometimes it’s only .001, As I stated above i will test and see if it affects my accuracy and ES.
I’m not saying crimping is no good, but I prefer not to do it unless i have to.

Will keep you updated.

 

[Chiquita]

FWIW, I experienced the same so I decided to measure jump with different factory ammo. And they all jumped when the bolt slammed home.

I will find my paper and post it here.

 

[mikecr]

I will increase neck tension to .004 and if not to .005 and see what happens. I will also test accuracy and ES with .003 neck tension that i have there now and see how it goes.

You're already sizing way into brass yielding both down and back up (at 3thou interference).
I'm assuming bushing sizing.
Seated bullets are just expanding necks back up, so I doubt that going even further to 4-5thou under cal will help.

This is a frictional matter, not tension. Otherwise I'd suggest stopping annealing, as this reduces tension.
You need enough friction to hold bullets,, to prevent slippage outward.
You said necks are washed. Are bullets washed (removing their sizing lube residue)?

Not a semi-auto shooter myself, and it's surprising to me that target SAs lack adjustable bolt closure damping. After all, there is no functional reason for fast closure with anything slower than full auto firing.

 

[Teflon]

You are correct i went .006 under and nothing changes. Bullet is still going forward. Took rifle to range today was getting ES of 7fps. And about .5 Moa group at 100 yards.

Will refine load development but i still did not stop bullet going forward with .006 neck tension.
Also send email to Seekins Precision to see what they got to say.

 

[Chiquita]

 

[Hohn]

The heavy weight of the big bcg is irrelevant to causing this issue, and adding more buffer weight could actually help.

The inertial force here is a function of cartridge velocity when it seats. A heavier bcg actually slows the acceleration a given spring force will generate, giving less velocity and less inertia to the cartridge. The impact force of the bolt and bcg is borne by the barrel extension, not the cartridge, at least for a *lengthening issue.

Personally, I think you’re making a big deal of nothing. If it’s not jamming the lands, it’s fine.

You might consider repeating the test with the same dummy round a dozen times and see if it continues or stops. This will help rule out measurement variation as calipers are about at their limits of resolution in the 0.001” range. (Note: technical resolution, not resolution in terms of numbers on the display.)

If you think this is an issue still, switch to a cannelure bullet, crimp it, and get on with life.

 

[243Mendoza]

Hello guys i have AR10.
Seekins Precision SP10 chambered in 6.5CM.
Lapua brass
130 Berger OTM

Im having issues bullet going forward when bolt takes round from magazine by .002 to .004 thousands. Thats right it goes forward between.002 to .004. My neck tension is .003 thousands. I tried loading factory ammo and same thing happened.

Any idea? Im trying to stay away from crimping.

how does that affect your groups? I doubt .004 affects accuracy when you load from the mag.

That is typical. I get much more movement.

 

[Metal God]

Annealing is the issue . I strongly recommend against fully annealing necks when using auto loaders . I’ve found heating necks to 750* to 850* stress relieves the necks enough to get better case life while still keeping a little more spring back for better neck tension. I’ve read you can do custom timing in the EMP but not sure how .

My guess is you can push the bullet deeper just by pushing the tip into your bench . If you can that’s to light of bullet hold for auto loaders IMO