Bullet Setback In AR

[Cfshooter]

Figure I would share some results from me messing around this morning. After checking for the lands in an AR barrel, I decided to check for any setback while letting bolt fly home. I was running about .002 neck tension and with bullet seated long (75BTHP.005 off lands-2.290 Coal) I sent bolt home 3 times and measured between each, not change to BTO. YMMV.

 

[jelenko]

Good data.
Just to confirm, bolt closure on single load rounds in an AR tends to cause the bullet to move out of the case.

 

[Cfshooter]

Good data.
Just to confirm, bolt closure on single load rounds in an AR tends to cause the bullet to move out of the case.

These were single loaded, halfway in chamber and then released bolt to fully chamber. I have found that if the bearing surface of bullet had complete contact inside of case neck, movement did not occur when sent home by bolt. Bullets seated long and that didn’t have complete contact with neck had different results.

 

[akajun]

Check the runout of the round before and after loading in an ar, especially from a magazine
My short line ammo I used a Lee factory crimp die
For long line just seated it

 

[jccinohio]

I had the opposite problem.. bullet would move forward after chambering. This was with 55gr vmax (zmax).. Essentially the BCG is an impact bullet remover: slams the round forward and then an abrupt stop. Bullets that are seated into the lands probably minimize the impact. I found I had to crimp to keep them from moving forward. After experimenting I found a pretty hefty crimp actually gave me my best groups. YMMV..

Same thing with my .308 AR and 110gr vmax..

 

[Cfshooter]

I had the opposite problem.. bullet would move forward after chambering. This was with 55gr vmax (zmax).. Essentially the BCG is an impact bullet remover: slams the round forward and then an abrupt stop. Bullets that are seated into the lands probably minimize the impact. I found I had to crimp to keep them from moving forward. After experimenting I found a pretty hefty crimp actually gave me my best groups. YMMV..

Same thing with my .308 AR and 110gr vmax..

Lol, yep, I have seen the same with bullets seated long and not having full contact of case neck.

 

[markm87]

Check the runout of the round before and after loading in an ar, especially from a magazine
My short line ammo I used a Lee factory crimp die
For long line just seated it

I like the Lee FCD die so much I added it to my long .223 ammo too.

 

[Ned Ludd]

I did a similar experiment some time ago in a Sig 556 piston gun, using non-cannelure bullets and non-crimped brass with necks sized to .002", .003", and .004" neck tension/interference fit. The bullet moved upon closure of the BCG in all three, but the movement was most consistent at .003" neck tension, so that's what I ultimately went with.

 

[Rocketvapor]

I like the Lee FCD die so much I added it to my long .223 ammo too.

With standard mandrel, Lee undersize, or sanded down mandrel?

 

[Cfshooter]

I did a similar experiment some time ago in a Sig 556 piston gun, using non-cannelure bullets and non-crimped brass with necks sized to .002", .003", and .004" neck tension/interference fit. The bullet moved upon closure of the BCG in all three, but the movement was most consistent at .003" neck tension, so that's what I ultimately went with.

were they seated long?

 

[Ned Ludd]

were they seated long?

Mag length, so basically the entire neck was engaged with bullet bearing surface. Both the magnitude and consistency of bullet movement upon BCG closure might well change if only part of the neck was engaged with bullet bearing surface (i.e. bullets seated much longer than mag length, or with fairly short/lightweight bullets). I think it's something that simply has to be tested to know, or else one could simply use cannelure bullets and crimp to largely avoid the issue. I didn't want to go that route for a number of reasons, which is why I did the test.

 

[markm87]

With standard mandrel, Lee undersize, or sanded down mandrel?

Necks are opened back up with a standard Redding (I think) neck expander die. The FCD is just set for a "light" crimp.

 

[Rocketvapor]

Sorry, my mistake. Thought is was the Lee Collet Neck die.

 

[drop_point]

The problem isn't the bullet setting into the case, but moving out of the case. I found this when I tried to load an ejected cartridge back into the magazine and it was too long. I started keeping track and found I needed either more neck tension or a crimp to prevent this. I settled on a Lee FCD as it shot better than adding more neck tension through the length of the case neck.

 

[GuitarmanNick]

My shooting may be less demanding than many here.
After some time and testing I have developed several loads that result in 1 MOA or better performance from all of my AR rifles.
I use the Lee FCD on every .223 round and never have an issue with setback or bullets pushing forward in my ARs. The consistency of the crimped rounds when fired from 3 different AR rifles(1:7 and 1:8 twist rates) is very good, too.

 

[dellet]

If the bullet is moving forward, longer, it’s happening during chambering.

If the bullet is moving backward, shorter, it’s happening in the magazine.

The exceptions would be if your load is a few thousands off the lands and the shoulder is also being set back allowing the bullet to hit the lands and push back. Or the tip is hitting something while feeding.

Recoil beats the nose of the bullet against the magazine.

Bolt speed and overall buffer, carrier, bullet weight is the controlling factor in the chamber. Mass and speed coming to an abrupt stop.

Consider that the system is designed to cycle with a shovel full of mud in the chamber or packed in ice at 40 below with frozen lube, it will move with more force than most people will ever need. Accuracy concerns are 2 MOA, but it better load and shoot that no matter what.

One thing I have found that helps, besides tuning the rifle to cycle a bit softer, is to use squeaky clean necks. The other is a sizing with a .002” under bullet diameter tool, ball mandrel, whatever. With case spring back it ends up .003-4”.

Consider a 300 Blackout with a 220 grain bullet and a case neck that is only about .250” long, and it’s a recipe for disaster.

The only thing that makes that worse, is a compressed load. Nothing like adding a spring underneath the bullet to help push it out when chambered.

 

[markm87]

Buffer and Bolt speed is another good point (touched on above). Many AR manufacturers over-gas their guns resulting in too much bolt speed. Combine that with a poorly matched buffer, and you can get bolt bounce.

Enough bounce, and your gun is an inertia bullet puller.

I've shot an AR-10 in 308 where the bolt bounced so bad it was distracting.

 

[Liven The Dream]

Well for years I shot Service Rifles and built many AR-15’s .
Used a RCBS SB Die to size and a Comp Seater .
Never saw this Problem?
Saw a few double feeds and pushed a bullet in ?
Think about Mag feed Bullets in a AR 15 having a Wylde Chamber ? 69gr. Sierra jumps a lot .

Good Luck