AR feeding problem 300BLK

[Crawfish1]

I have a 300 Blackout I am having feeding problems using Missouri Cast 300 Blackout - Hi-Tek .309 Diameter 245 Grain RNFP.

I can manually load a shell by hand and let the bolt release and fire with normal extraction. If I put the same round in a magazine sometimes I will get 2 to cycle and then it will malfunction. Sometimes I won't have a problem for 15 rounds.

When the bolt fails to close on a round being stripped off of a magazine I can't pull the charging handle back to eject the round. I have to push the charging handle up against something like a 6x6 and push on the butt stock to get it to eject. I can pick up the round that had just been stuck and put it in the chamber release the bolt and it will fire/eject normally.

I have tried three different brand magazines. Ideas?

 

[C-grunt]

Maybe the RNFP profile jamming while feeding?

 

[Walkalong]

Measure the neck of a loaded round.

 

[crazysccrmd]

Check that you are bumping the shoulder back far enough.

Check that you aren't jamming the bullet into the lands.

Check neck diameter when loaded compared to a fired casing.

Check that you aren't bulging the shoulder when seating.

 

[TIMC]

Agreed on the bullet profile. With such a large bullet I can see feeding issues.

 

[Crawfish1]

Ok,

Measured some of my reloads and neck of cases unfired are measuring .292
Base of cases are measuring up to .375


Next time I get a jam I will take the calipers and measure it. Crazysccrmd what do you mean by bumping the shoulder?

 

[crazysccrmd]

When you fire a round the case grows in length slightly. If you don't have the sizing die set right to push the shoulder back down the case it will prevent the round from chambering properly and jam up in the chamber when forced into battery. It's a very small bump back but enough to make a huge difference.

 

[Tirod]

It's also a round nose cast lead bullet. That isn't an optimal feeding profile for a self loading action - it's a manual action type bullet.

The cartridge coming off the magazine has to point upwards at the chamber and the nose hits the feed ramps and the interior of the chamber wall to straighten out. A typical spire point has room to do that, a round nose is much blunter and smacks head on into things. It doesn't slide off like a brass jacketed round will do.

The AR action was designed for tapered spire points.

 

[crazysccrmd]

It's also a round nose cast lead bullet. That isn't an optimal feeding profile for a self loading action - it's a manual action type bullet.

The cartridge coming off the magazine has to point upwards at the chamber and the nose hits the feed ramps and the interior of the chamber wall to straighten out. A typical spire point has room to do that, a round nose is much blunter and smacks head on into things. It doesn't slide off like a brass jacketed round will do.

The AR action was designed for tapered spire points.

That's true, but he said it jams up in the chamber, not on the feed ramps. It sounds like his rounds are making it up the feed ramps and into the chamber before the problem happens.

I've had the exact same problem with a round chambering but not locking and had to mortar the action open like he describes. I was experimenting with how long I could get away with just neck sizing in an AR without a full length size to bump the shoulder back.

 

[jmorris]

Might try a jacketed bullet, some just don't like cast bullets.

 

[Walkalong]

Measured some of my reloads and neck of cases unfired are measuring .292

If that is a loaded round, you need to check your calipers.

If not, and it's just a sized case, it is meaningless. You need to measure the neck on a loaded round to make sure it isn't too big for the chambers neck area.

The question of sizing the case correctly is also key.

 

[Walkalong]

Check that you are bumping the shoulder back far enough.

Check that you aren't jamming the bullet into the lands.

Check neck diameter when loaded compared to a fired casing.

Check that you aren't bulging the shoulder when seating.

Check any of these yet?

 

[Crawfish1]

Walkalong,

You were right my digital calipers did not read the same as my mechanical ones. The readings were .331 unfired and .335 fired. More interesting is the fact I got the rifle to run perfectly with a Lancer magazine. I shot 50 rounds through the Lancer without issues or any problems. I put a Magpul in and immediately the gun became a jam-omatic again. Put the Lancer in and no more problems. My rifle must be mag sensitive as I have used three other makes to no avail. Thanks everyone for the help. I have been fighting this rifle for months and now it seems I have a resolution.