After searching the web diligently for the actual differences between DPMS and Armalite 308 BCG's I thought I would document my experience.
This all started with my desire to use the smaller Armalite bolt firing pin (.068) to resist primer piercing when loading slow burning powders, and take advantage of the supposedly stronger balanced bolt lug design of the Armalite bolt with a recent 260 Remington upper build.
I've been piecing this build together and used a Lilja 260 Rem barrel (DPMS extension) in a new DPMS A3 upper. I already have a complete DPMS LR 308 rifle with a Rainier Select 20" 308 barrel, DPMS extension. I found a new complete Armalite BCG for a decent price, and the following documents what I found about DPMS vs Armalite BCG dimensional compatability.
In my research I found examples of builds using the complete Armalite BCG with DPMS extensions successfully. I wanted to determine if the Armalite bolt or BCG were safe using a DPMS barrel extension.
I have headspace gauges for the 308 cartridge family in GO 1.630, NO GO 1.634, and FIELD 1.638 dimensions.
After stripping both bolts by removing the extractor and ejector parts, I compared the bolts, bolt components, and carriers. The bolt carriers, bolts and bolt components appeared identical to the naked eye.
I then proceeded to insert the GO gauge in the 260 chamber, followed by the Armalite BCG with stripped bolt. It closed with no resistance on the GO gauge. My first problem began when I tried to manually cycle the BCG. It retracted about a half inch and seized up very tight. The BCG extended far enough into the lower buffer tube that I couldn't separate the upper from the lower even with both takedown pins withdrawn. Nor would it cycle forward into battery.
WTF? After an hour or so of trying to separate the upper from the lower, I noticed the DPMS hammer seemed to be stuck in the bolt hammer notch in the bottom of the Armalite bolt carrier. After using a small flatblade screwdriver to depress the hammer through the gap between the upper and lower with both pins out, I was able to remove the Armalite BCG from the DPMS upper and separate the halves.
When I finally got the them apart, I discovered what an Armailte narrow hammer BCG looks like. Evidently the older (pre 2003 or so) bolt carriers used a .300 or so thick hammer, vs a regular .495 or so DPMS AR hammer. The DPMS AR hammer WILL enter the bolt slot enough to lock shit up as described above.
OK, swap out the factory trigger group in my lower for a RRA two stage I had laying around that has a narrow dimension hammer. Now we're cooking. BCG cycles fine.
Observation #1. DPMS lower, DPMS A3 upper, 260 Rem Lilja barrel, DPMS extension.
Armalite stripped bolt with Armailte carrier, closes on Go, closes on No Go, closes on Field. Not good. Estimated headspace 1.638+.
DPMS stripped bolt with DPMS carrier, closes on GO, will not close on NO GO or FIELD. Estimated headspace 1.632.
Armalite stripped bolt in DPMS carrier, closes on GO, closes on NO GO, closes on Field. Estimated headspace 1.638+.
DPMS stripped bolt in Armalite carrier, closes on GO, closes on NO GO with resistance, will not close on Field. Estimated headspace 1.635.
Observation #2 DPMS lower, DPMS A3 upper, Rainier Select 308 barrel, DPMS extension.
Armalite stripped bolt with Armailte carrier, closes on Go, closes on No Go, closes with resistance on Field. Estimated headspace 1.637.
DPMS stripped bolt with DPMS carrier, closes on GO, will not close on NO GO or FIELD. Estimated headspace 1.632.
Armalite stripped bolt in DPMS carrier, close on GO, closes on NO GO, closes with resistance on Field. Estimated headspace 1.637.
DPMS stripped bolt in Armalite carrier, closes on GO, closes on NO GO with resistance, will not close on Field. Estimated headspace 1.635.
Conclusions;
There are apparently dimensional differences between the Armalite and DPMS bolts of around .004 to .006 in headspace, with the Armalite being longer with a DPMS barrel extension. I would also guess the bolt extensions (which are stated to be the same) are installed differently to allow the Armalite bolt to headspace properly with an Armalite extension. You may get away with mixing and matching original firings of virgin brass, but without adjusting your reloading practices to allow for the headspace stretching issues, case separations will be unavoidable.
Hope this helps others, and don't try this experiment with the older narrow hammer Armalite BCG in a lower with a full dimension AR hammer. It really is a major bitch until you figure it out.
This all started with my desire to use the smaller Armalite bolt firing pin (.068) to resist primer piercing when loading slow burning powders, and take advantage of the supposedly stronger balanced bolt lug design of the Armalite bolt with a recent 260 Remington upper build.
I've been piecing this build together and used a Lilja 260 Rem barrel (DPMS extension) in a new DPMS A3 upper. I already have a complete DPMS LR 308 rifle with a Rainier Select 20" 308 barrel, DPMS extension. I found a new complete Armalite BCG for a decent price, and the following documents what I found about DPMS vs Armalite BCG dimensional compatability.
In my research I found examples of builds using the complete Armalite BCG with DPMS extensions successfully. I wanted to determine if the Armalite bolt or BCG were safe using a DPMS barrel extension.
I have headspace gauges for the 308 cartridge family in GO 1.630, NO GO 1.634, and FIELD 1.638 dimensions.
After stripping both bolts by removing the extractor and ejector parts, I compared the bolts, bolt components, and carriers. The bolt carriers, bolts and bolt components appeared identical to the naked eye.
I then proceeded to insert the GO gauge in the 260 chamber, followed by the Armalite BCG with stripped bolt. It closed with no resistance on the GO gauge. My first problem began when I tried to manually cycle the BCG. It retracted about a half inch and seized up very tight. The BCG extended far enough into the lower buffer tube that I couldn't separate the upper from the lower even with both takedown pins withdrawn. Nor would it cycle forward into battery.
WTF? After an hour or so of trying to separate the upper from the lower, I noticed the DPMS hammer seemed to be stuck in the bolt hammer notch in the bottom of the Armalite bolt carrier. After using a small flatblade screwdriver to depress the hammer through the gap between the upper and lower with both pins out, I was able to remove the Armalite BCG from the DPMS upper and separate the halves.
When I finally got the them apart, I discovered what an Armailte narrow hammer BCG looks like. Evidently the older (pre 2003 or so) bolt carriers used a .300 or so thick hammer, vs a regular .495 or so DPMS AR hammer. The DPMS AR hammer WILL enter the bolt slot enough to lock shit up as described above.
OK, swap out the factory trigger group in my lower for a RRA two stage I had laying around that has a narrow dimension hammer. Now we're cooking. BCG cycles fine.
Observation #1. DPMS lower, DPMS A3 upper, 260 Rem Lilja barrel, DPMS extension.
Armalite stripped bolt with Armailte carrier, closes on Go, closes on No Go, closes on Field. Not good. Estimated headspace 1.638+.
DPMS stripped bolt with DPMS carrier, closes on GO, will not close on NO GO or FIELD. Estimated headspace 1.632.
Armalite stripped bolt in DPMS carrier, closes on GO, closes on NO GO, closes on Field. Estimated headspace 1.638+.
DPMS stripped bolt in Armalite carrier, closes on GO, closes on NO GO with resistance, will not close on Field. Estimated headspace 1.635.
Observation #2 DPMS lower, DPMS A3 upper, Rainier Select 308 barrel, DPMS extension.
Armalite stripped bolt with Armailte carrier, closes on Go, closes on No Go, closes with resistance on Field. Estimated headspace 1.637.
DPMS stripped bolt with DPMS carrier, closes on GO, will not close on NO GO or FIELD. Estimated headspace 1.632.
Armalite stripped bolt in DPMS carrier, close on GO, closes on NO GO, closes with resistance on Field. Estimated headspace 1.637.
DPMS stripped bolt in Armalite carrier, closes on GO, closes on NO GO with resistance, will not close on Field. Estimated headspace 1.635.
Conclusions;
There are apparently dimensional differences between the Armalite and DPMS bolts of around .004 to .006 in headspace, with the Armalite being longer with a DPMS barrel extension. I would also guess the bolt extensions (which are stated to be the same) are installed differently to allow the Armalite bolt to headspace properly with an Armalite extension. You may get away with mixing and matching original firings of virgin brass, but without adjusting your reloading practices to allow for the headspace stretching issues, case separations will be unavoidable.
Hope this helps others, and don't try this experiment with the older narrow hammer Armalite BCG in a lower with a full dimension AR hammer. It really is a major bitch until you figure it out.
