AR bolt not closing all the way

[deemus]

Anyone ever have this issue?

I bought a 458 SOCOM upper. I am using a metal GI mag.

If the mag is in the gun, the bolt won't close all the way. I remove the mag, and re-rack the bolt, and it closes all the way.

Anyone know why this happens?

 

[AZ Refugee]

Magazine sets too high, try a P-mag and see it is works.

 

[deemus]

I'll try that. Generally Pmags are not recommended for 458 SOCOM, so I didn't try one.

 

[AZ Refugee]

I built a long range AR from an existing Bushmaster lower and bought a stripped upper from Bushmaster, It was marked .450. I do not know if it was off, or if the new bcg was a little different, but it did not like the GI mags, but runs like a champ with Pmags.

 

[deemus]

I called the manufacturer, and they said they use Lancer L5 mags from CMMG when they test fire their 458 rifles. I have two on order. Hoping that fixes it. I know the mag can be an issue for the 458.

 

[grasshopperglock]

The 450 bushmaster has the same issue.

On an open bolt, mag inserted. Bolt released to charge it. It'll jam a round going up the feed ramp.

Mag inserted on a closed bolt. Charged to load chamber and it works fine. Common Bushmaster magazine problem.

 

[deemus]

The 450 bushmaster has the same issue.

On an open bolt, mag inserted. Bolt released to charge it. It'll jam a round going up the feed ramp.

Mag inserted on a closed bolt. Charged to load chamber and it works fine. Common Bushmaster magazine problem.

My round feeds, but the bolt won't close.

 

[grasshopperglock]

My round feeds, but the bolt won't close.

The 450 feeds but the bolt doesn't go into closed battery.

 

[deemus]

Finally figured it out. Had to run the rounds through the Lee crimping due, but very light crimp. They chambered and cycled as intended.

This round is pretty finicky. But I think I have the process down now. The CMMG modified Lancer mags definitely helped too. They are modified for 458 SOCOM, and even have "458 SOCOM" printed on the mag.