PMP brass not extracting from DM4

**Armed Preacher** · 20-04-2017, 18:13

Originally Posted by M43

Does anyone on this forum know what length gas system is used on an 18 inch DM 4?

Do they have rifle and or Mid-length gas systems available on the longer barrels or do those use the carbine gas system?

Carbine length in all DM4

**Armed Preacher** · 20-04-2017, 18:16

Originally Posted by Mozzy

Thanx for your reply Armed Preacher. There is indeed a second round, fed from the mag, jammed against the case in the chamber.

Do the extractor spring/o-ring upgrade and all should be good.
You could also replace Reg bolt carrier to a full-auto type or upgrade the buffer to H2, both will slow the system down sufficiently to allow reliable extraction.

**Lian** · 20-04-2017, 19:24

Surely if the bolt requires something as critical as a mere oring for reliable extraction, it would fall under some kind of warranty?

**M43** · 20-04-2017, 20:19

Originally Posted by Armed Preacher

Carbine length in all DM4

Perhaps that would explain some of the problems with PMP factory ammo?. Normally a carbine length gas system is fed gas for the 7.5 inches of barrel in front of the gas port (on a 14,5 inch barrel), and then pressure drops off. An 18 inch barrel with a carbine gas system has 11 inches of barrel time providing gas to the system. Paul mentioned over-gassing could be an issue, I am inclined to agree with this possibility.

Normally the gas system on an AR is timed to allow the bolt to start moving back as soon as pressure drops below a certain safe level. The extra 4 inches of barrel on a 18 inch barrel with a 7 inch gas system is going to cause pressure to build up and stay high for a much longer time.

I don't build AR's for a living but I believe Colt, FN, or any of the major manufacturers do not put carbine gas systems on 18 inch or longer barrels, for good reason.

**TRG** · 20-04-2017, 20:21

Originally Posted by Gleock

The change at the shoulder is irrelevant and there is more variation between different makes of .223 reamers, than between 5.56 and .223.
The major difference is in the length and angle of the throat. That has no effect on how the cartridge headspaces.

If your rifle has an extraction issue due to a rough chamber ,polishing might help,
But if the chamber hasn't been well cut by the factory , re-cutting the chamber with any other 223/5.56 reamer in the same chamber would definitely change the headspace.
One of our club members bought a brand new Ruger AR15 DI rifle.
It failed to extract on all ammo.
Examining the fired cases it revealed a constant protrusion in the brass body. ( bore scope confirmed a faulty chamber. )
In the USA factories would just replace the barrel.

**Mozzy** · 28-04-2017, 00:10

Hey guys. I trust you're well. Just to let you know that I resolved the issue with my DM4 not extracting PMP brass. I fitted the O ring as suggested by SafeAction and it works excellently. A quick cheap fix and I'm waxed. Thanx SafeAction for you help and thank you all for you replies.
God bless

**Wanderin' Zero** · 30-04-2017, 06:47

Mozzy any idea what age / date of manufacture the PMP ammunition is?

http://www.gunsite.co.za/forums/show...mp-round-fired

**Mozzy** · 01-05-2017, 20:45

No idea Wonderin'. There's no indication on the packaging of batch/lot date of manufacture etc.

**Schmidtj** · 30-08-2018, 13:15

Originally Posted by Mozzy

Hey guys. I trust you're well. Just to let you know that I resolved the issue with my DM4 not extracting PMP brass. I fitted the O ring as suggested by SafeAction and it works excellently. A quick cheap fix and I'm waxed. Thanx SafeAction for you help and thank you all for you replies.
God bless

Hi Mozzy i recently got my DM4 all is working perfectly on it but thought is should ask you what size is the o-ring that you fitted? And where can i get it ?

**FNBROWNING** · 30-08-2018, 14:45

Originally Posted by Gleock

The change at the shoulder is irrelevant and there is more variation between different makes of .223 reamers, than between 5.56 and .223.
The major difference is in the length and angle of the throat. That has no effect on how the cartridge headspaces.

TRG has a valid point. As you mention there are variations between different reamers. All of which alters chamber specs. Then there are potential alignment issues when it comes to cutting and or polishing. Apparently it is not uncommon for cut chambers to not be totally aligned with the bore, more so I would imagine in one off variations. In this regard and especially for machine guns (for info purposes) bore chamber centricity is paramount and hence ideal is hammer forging both at the same time.