Bullet seating tests in vain with Semi-automatic AR-10

Solidbuilt

Solidbuilt

Private
Minuteman
Nov 10, 2024
19
5
Hawaii
#1
I just discovered something about bullet seating and "reverse" setback or "bullet jump":

When I mandrel expand my cases to .002 neck tension, and then seat my bullet with a crimp (Lee Factory die crimp), the bullet jumps forward. 005 (and higher) after I close (slam) the bolt on my AR-10!

So, all my seating tests have been in vain.

I tested several bullets this morning, all starting with a COAL of 2.8... then, after chambering the rounds, I removed them from the chamber and remeasured- all seating depths changed drastically to 2.805, 2.81, and some even higher.

I may have to change my neck tension to. 004 because the ones I tried at .003 also jumped out further.

I even gently loaded a few to make sure the chamber wasn't grabbing on extraction, and it's not.
Slamming the bolt home causes my bullet seating to change drastically at .002 and. 003 even with the crimp.

So, if I increase my neck tension to .004, which many people online recommend for semi's, then it'll take more force to seat the bullet which comes with its own slight degradation in run out and concentricity... right?

What do you guys think about all of this?
 
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#5
I think the easiest solution would be add some more crimp rather mess with the neck tension. For a pure accuracy standpoint add more neck tension and see if you can skip the crimp step. It was my understanding that the lee factory crimp was more like a taper crimp die therefore eliminating the need for a cannelure as with a roll crimp. Am I wrong about that?
 
#6
rweaver00geo said:
I think the easiest solution would be add some more crimp rather mess with the neck tension. For a pure accuracy standpoint add more neck tension and see if you can skip the crimp step. It was my understanding that the lee factory crimp was more like a taper crimp die therefore eliminating the need for a cannelure as with a roll crimp. Am I wrong about that?
Click to expand...
The Lee Factory Crimp die does not need a cannelure.... but, it's somewhat different than a taper crimp.... its supposed to be better....
 
#7
You have discovered that a Semi Automatic rifle is a different animal than a bolt action rifle. When the round hits the shoulder of the chamber the case stops but the inertia of the bullet keeps it moving. You really can't stop it from happening. Even cannelured bullets will move a few thousandths of an inch. You simply have to account for it.
 
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#8
I will add that after thinking about your post you are not letting the bolt release close the bolt on the test rounds. If that is the case I would seriously recommend that you consider doing so. Not allowing the buffer spring to close the bolt means you are riding the BCG into battery. It is possible in that case that the bolt does not fully rotate closed leaving the lugs only partially engaged. In theory the hammer isn't supposed to strike the primer if this happens but that is a very fine line that I wouldn't bet on..

Also, if your seating depth is so sensitive that a 2 or 3 thousandth variation takes it out you need to look for a better seating depth.

As for neck tension, you can't realistically eliminate the bullet setting itself forward.
 
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#11
I started running 4 thou neck tension on all my gas gun loads and found more consistency and accuracy. and the last few I've found seating a little deeper in the case seemed to yield better results also. I played with crimping for a while also, but wasn't finding what I was looking for. I started making it a point when I'm doing loads for gas guns to always measure any movement of the bullet on a full send chambering to double check and verify any bullet movement.
 
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#16
Throwing another 2 cents into the hat here, but I've started wondering a few times if the amount of bearing surface of a particular bullet has an effect that isn't being taken into account. And also the amount of seating depth of that bearing surface into the case neck. When my ar10 was 308 I found I never noticed any issues shooting the traditional 150 ish grain standard profile bullets, but my gun hated them and I moved to 185 jugs. That's where I noticed it the most. In my head my thought process was because there was so much more mass of the bullet outside the case neck that it was having more effect. Especially if seating long and not having a decent amount of bearing surface in the case neck. IDK . Food for thought I guess...
 
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#17
I've found my work up... 2 range sessions in a row with identical results.... 185gr Juggernauts/44.1gr Varget seated at 2.79, expanded necks with .0025 expander, and light crimp with Lee Factory Crimp Die....sub .75 MOA.... 4.1 SD.... (as a reminder- this load is over book max and is probably not safe for your rig. I do not recommend this, or any, loads...I'm just reporting for informational purposes)... shooting out of an 18" Bartlein barrel AR-10 turned by Craddock Precision.
 

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#19
Bobby Ball said:
Throwing another 2 cents into the hat here, but I've started wondering a few times if the amount of bearing surface of a particular bullet has an effect that isn't being taken into account. And also the amount of seating depth of that bearing surface into the case neck. When my ar10 was 308 I found I never noticed any issues shooting the traditional 150 ish grain standard profile bullets, but my gun hated them and I moved to 185 jugs. That's where I noticed it the most. In my head my thought process was because there was so much more mass of the bullet outside the case neck that it was having more effect. Especially if seating long and not having a decent amount of bearing surface in the case neck. IDK . Food for thought I guess...
Click to expand...
The amount of bearing surface absolutely influences the amount of force required to move a bullet. It's easy to demonstrate if you have an impact bullet puller. Seat on with a short bearing surface ( .25 x caliber) in the mouth and one at 1 caliber and try getting the bullet out!
 
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