AR10 Neck tension Issues

[Teflon]

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.

 

[Sniperhandle]

I always run my Auto-Loader rounds through a Lee Factory crimp die. Otherwise you may experience the bullets shifting like you have.

I don't shoot any match grade stuff out of my AR's, I am a hard core bolt gun guy.

 

[otnot]

I taper crimp all my semi auto reloads

 

[stavey]

Increase neck tension and repeat the test . Also check to see if the bullet is slamming into the chamfer or the area below the chamfer , before being forced into the bore. Causing the setback. Barrel on the right is a proof on the left is a bart. Notice the larger chamfer around the proof bore. The proof barrel feed beautifully. The Bart does not. Just something to look at.

 

[Teflon]

Increase neck tension and repeat the test . Also check to see if the bullet is slamming into the chamfer or the area below the chamfer , before being forced into the bore. Causing the setback. Barrel on the right is a proof on the left is a bart. Notice the larger chamfer around the proof bore. The proof barrel feed beautifully. The Bart does not. Just something to look at. View attachment 1377099View attachment 1377100

Awesome pictures there and thank you!
My bullet gets pushed forward i to barrel not backwards into case. That’s why I'm confused .

Guys thank you for trying to help.

 

[DShortt]

How are you determining that your bullets are moving forward exactly? Not saying you are wrong this just sounds highly unusual. Also, how exactly are you measuring for neck tension? The only thing I can think of off hand that might allow this to happen is if the neck tension is too light.
As others have said I also crimp all my semi-auto loads to prevent such an occurrence.

 

[Rocketvapor]

Happens with factory rounds?

 

[DShortt]

Happens with factory rounds?

Something isn't right.

 

[Teflon]

So i will load dummy round that is sized down and neck expended to be .003 for example .261 since bullet is .264. Than i would measure it ogive 2065of dummy round and load round in magazine and release bolt to chamber round. Slowly extract round and remeasure it and i would find to be at 2067 or 2068 if i do same thing with that dummy round and load it one more time from mag i will grown another .002. So bullet would move forward .002 or .003 again

 

[Teflon]

Happens with factory rounds?

Yes same with factory Match ammo

 

[Rocketvapor]

Load a dummy round and try and pull the bullet/twist with your hand.

 

[DShortt]

So i will load dummy round that is sized down and neck expended to be .003 for example .261 since bullet is .264. Than i would measure it ogive 2065of dummy round and load round in magazine and release bolt to chamber round. Slowly extract round and remeasure it and i would find to be at 2067 or 2068 if i do same thing with that dummy round and load it one more time from mag i will grown another .002. So bullet would move forward .002 or .003 again

Ok, 2 questions. First, you've checked this with factory ammunition and found it to be consistent? And 2, you've probably got a lot more than a few thousandths between the ogive and the lands unless this is a chamber with next to no freebore so what issue is this causing for you apart from an insignificant change in CBTO that you can only see if you check it with a comparator?
I honestly don't know if my AR-10 does this or not because I don't recall checking for anything but set-back and not often because I crimp my semi-auto reloads so I'm confused here.....

I re-read my post and it sounds a bit snarky so I apologize for that. I am a bit confused about if this is something to really be concerned about though. Set back is an obvious issue, 'set forward'? First I've ever heard about this.

 

[mikecr]

Are you annealing the necks?
What is the frictional condition of bullets and necks?

 

[Sniperhandle]

What happens when a magician pulls the table cloth out from under the item on top? The table cloth is moving backward and the items stay put, Same thing is happening with your bullets with a light or no seating crimp.

 

[Jeff Porter]

Not surprising if you have ever held a big AR and dropped that heavy azzed bolt and felt it slam home.

When the case hits the shoulder in the chamber the bullet has a lot of energy moving forward. it will jump out until it hits the rifling if you dont have enough tension. ( an object in motion will stay in motion...)

You need a heavy neck tension to keep an OAL the same when feeding those gas guns from a magazine. If I am doing precise work with a big gas gun I will single load through the port and push it in to the chamber before closing the bolt.

I sometimes chuckle when some one says they are loading to a specific jump in a gas gun then load from the magazine or a SLED and let the bolt send it home. You need a lot of neck tension or a crimp to keep it there.

 

[wwbrown]

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

 

[robert8904]

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.

 

[Teflon]

Ok, 2 questions. First, you've checked this with factory ammunition and found it to be consistent? And 2, you've probably got a lot more than a few thousandths between the ogive and the lands unless this is a chamber with next to no freebore so what issue is this causing for you apart from an insignificant change in CBTO that you can only see if you check it with a comparator?
I honestly don't know if my AR-10 does this or not because I don't recall checking for anything but set-back and not often because I crimp my semi-auto reloads so I'm confused here.....

I re-read my post and it sounds a bit snarky so I apologize for that. I am a bit confused about if this is something to really be concerned about though. Set back is an obvious issue, 'set forward'? First I've ever heard about this.

No I appreciate your help and honesty.
I was asking is this normal for AR10. It’s brand new SP10 so i will do load development and see if this is issue or not. If gun performs good and i can keep it with 1moa or better at 100 yards will ignore it.

 

[Teflon]

Are you annealing the necks?
What is the frictional condition of bullets and necks?

Yes with Annealing made perfect. Everything looks good and normal when i extract round. No marks on bullet or neck.

 

[Teflon]

Not surprising if you have ever held a big AR and dropped that heavy azzed bolt and felt it slam home.

When the case hits the shoulder in the chamber the bullet has a lot of energy moving forward. it will jump out until it hits the rifling if you dont have enough tension. ( an object in motion will stay in motion...)

You need a heavy neck tension to keep an OAL the same when feeding those gas guns from a magazine. If I am doing precise work with a big gas gun I will single load through the port and push it in to the chamber before closing the bolt.

I sometimes chuckle when some one says they are loading to a specific jump in a gas gun then load from the magazine or a SLED and let the bolt send it home. You need a lot of neck tension or a crimp to keep it there.

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. But i will do one change at the time and will update you guys here.