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Posted: 6/29/2015 11:34:46 AM EDT
Hi All

I am at my end with this rifle . I need some help please .
The DM4 is a locally built AR platform here in South Africa , I purchased the very first one in 300 AAC BLK .
Mine came with a 12,5 inch barrel . It has a 1 in 12 twist .It works well with anything up to about a 150gr round over that they tumble and connect side on .
It is not suppressed and was  not intended for such. She has an adjustable stock .
Very accurate with 110 VMAX at 2100fps

My problem ....The rifle will not cycle anything that runs under 2100FPS irrelevant of bullet weight .
I would like to use it on lighter loads from about 1200fps . I have ensured that the port is clear , tried shorter springs ( 2 inch shorter )
I have removed all weight from the carbine buffer

Please I would appreciate any advice with this

Thanks
Lloyd - Darkest Africa
Link Posted: 6/29/2015 12:19:18 PM EDT
The twist rate is your issue.  You need a 1-7 or a 1-8 to run subs.  Does it also have a carbine gas system?  If so, you may have met your max at the 150gr mark.  

.300blk was designed for running in a 9" barrel with pistol gas system and a suppressor.  Supers are a nice addition, but you will have problems with subs and no can.
Link Posted: 6/29/2015 12:43:28 PM EDT
Hi

So if I understand you correctly it will only cycle ammo that runs under 2100fps if it has a pistol length gas system and a can ?

So currently the carbine system is too long . Currently the barrel port is situated roughly 8 to 10 inches ahead of the breech .
Link Posted: 6/29/2015 2:53:59 PM EDT
Link Posted: 6/29/2015 4:54:08 PM EDT
Discussion ForumsJump to Quoted PostQuote History
Quoted:
Hi

So if I understand you correctly it will only cycle ammo that runs under 2100fps if it has a pistol length gas system and a can ?

So currently the carbine system is too long . Currently the barrel port is situated roughly 8 to 10 inches ahead of the breech .
View Quote



You can run subs in a 16" with a carbine setup, if you have the right twist rate and a can on the end to help with the pressure.  But your problem is solely the twist rate.  It is not set up to run heavier sub bullets.
Link Posted: 6/29/2015 5:25:11 PM EDT
You would be well served going to a lightened carrier as well, but as others have stated you need a faster twist rate to the barrel to stabilize longer projectiles.  The lightened reciprocal mass will help with lighter charges operating the system, and you may need more gas as well (I'm assuming you have a pistol-length gas system which is almost a necessity).  But yes, it appears the builder lacked understanding of the cartridge, and therefore the necessary twist rates to give you the widest range of functional projectiles.
Link Posted: 6/30/2015 5:57:20 AM EDT
Thanks Guys , you are right Brian , they build 223 , but this was the first 300 they built and they admitted to still be "learning " on the 300 ....I don't mind this odd ball , especially because its the very first DM-4 in 300 Blk .

It would just be nice to experiment with a slightly wider range of powder weights in the loads . The powder I am using is a local powder called S265 and I need a minimum of 16grain to achieve the 2100fps to get the rifle to cycle the ammo . AT 16grain the case is virtually full . It would be nice to play with lighter loads .

I am thinking of fitting a can now just to get it to cycle the lighter loads ...any ideas around this ?

Thanks LLoyd ....sitting in the bush here
Link Posted: 6/30/2015 5:36:45 PM EDT
1x12 twist is for light bullets............1x7 and 1x8 will shoot all.......heavy bullet = fast twist...........
Link Posted: 6/30/2015 5:55:40 PM EDT
I bet it will cycle supersonic ammo just fine if they get the port size opened up to where it needs to be.

I would want a new barrel in 1/8 twist if I were you, but if you can live with lightweight bullets at full power

they can get it to cycle.
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