300 rcm in a ar-10

[assasinator]

so. i have formed 50x300RCM out of 7mm PRC. they spec out exactly right.

i also found 60 rounds of hornady 178 VLD and i have them in hand. i like the 2900fps box, but i will chronograph them.

i chose this round to give my ar-10 a magnum .300". i also chose it because it is the closest diameter (.532) and brass dimensions to my 308.

i have a variety of metal and pmags. 5, 10 ,20, 25.

i also have 300 RCM reamer and go-no go gauge. 10 twist, 20" barrel with rifle length gas block.

so, i bought sand rolls to modify or remove the guide that i see the RCM hitting. if i am right and the RCM fits, i will get the barrel reamed.

IF THERE IS a magnum 300 that will work in a DPMS(AERO) ar-10, it is the 300 RCM.

i am grinding the 20 round PMAG first. it needs .100" clearance to where the brass will not hit the guide.

i also have a .534" KAK bolt for my ar10 in hand.

 

[Zen Archery]

Dead Shot Barrels has a 338 RCM that runs on a standard AR10 lower.
I am running a 300 WSM and and 358Win on a standard AR10 lower.
I don't believe you'll have any issues with 300 RCM. Biggest consideration is magazine to bullet length.
For my 300 WSM I had to cut the face out of the magazine for the bullets to fit properly.

 

[cohunt]

300wsm, 300 rsaum, 300rcm all seem to be peas in a pod to me

 

[ARlife4me]

since the 300rcm is the parent case for 6.5prc (iirc?) you'll need a mag with a better follower (or modified)? i have a 6.5prc and 7saum in AR10's, but use asc and larue mags as they will give a longer oal the pmag m118's. if you want to stay @ 2.830" or less than, you won't need them? any dpms/sr25/lr308 lower will do and upper ejection port will need to be enlarged (lower portion as to keep cover on) by 50-100thou.

 

[ARlife4me]

get an extension that is a single feed as the brass will sit in the middle and not staggered like 308 and 556 does. from my experience with short mag cartridges you'll only get about 60% of normal capacity due to follower issues. 10 round mag will be the better choice and 20 rounder might get you 8-12 before swelling the mag making it difficult to insert.

 

[assasinator]

Dead Shot Barrels has a 338 RCM that runs on a standard AR10 lower.
I am running a 300 WSM and and 358Win on a standard AR10 lower.
I don't believe you'll have any issues with 300 RCM. Biggest consideration is magazine to bullet length.
For my 300 WSM I had to cut the face out of the magazine for the bullets to fit properly.

so i realize that the rcm,wsm,and saum will shoot in a ar-10.

its the magazine capacity i'm NOT willing to lose, accounting for .532" vs. .473".

i have the parts to run the 300rcm, my gunsmith will ream my 308 to 300rcm.

any round shorter or equal to the 308 will cycle a few rounds. i want 10 or more in a 20 round mag.

i tried loading 300 rcm into my unmodded steel and pmags. they will not work without modding the guide rail similar to a 300 blackout/ar-15 pmag.

this thread is really about getting pmag capacity. the taper of the 300 rcm is close enough IMO to run in a 308 mag ONLY if sand rolling the rail works.

when you say cut out the face, what does that mean? any help getting capacity is appreciated. my other gunsmith is the one who mentioned modding it like 300BLK/ar15.

thanks- bill

 

[assasinator]

300wsm, 300 rsaum, 300rcm all seem to be peas in a pod to me

the rcm is .532" and thats a LOT to me when trying to get magazine stacked capacity.

my bolt is .534"

 

[assasinator]

since the 300rcm is the parent case for 6.5prc (iirc?) you'll need a mag with a better follower (or modified)? i have a 6.5prc and 7saum in AR10's, but use asc and larue mags as they will give a longer oal the pmag m118's. if you want to stay @ 2.830" or less than, you won't need them? any dpms/sr25/lr308 lower will do and upper ejection port will need to be enlarged (lower portion as to keep cover on) by 50-100thou.

i will be looking up your suggestions. i load them, so keeping 308 COAL isnt a issue. i ALSO form my brass from 7mm PRC.

 

[assasinator]

get an extension that is a single feed as the brass will sit in the middle and not staggered like 308 and 556 does. from my experience with short mag cartridges you'll only get about 60% of normal capacity due to follower issues. 10 round mag will be the better choice and 20 rounder might get you 8-12 before swelling the mag making it difficult to insert.

swelling is the issue. i will just keep the 308 if i cant get capacity, even though i have all of the parts to drop into my ar10.

 

[vern.284]

Tag

 

[ARlife4me]

the rcm is .532" and thats a LOT to me when trying to get magazine stacked capacity.

my bolt is .534"

It's fine as you can use .532, .534 and .535 cases

 

[ARlife4me]

swelling is the issue. i will just keep the 308 if i cant get capacity, even though i have all of the parts to drop into my ar10.

Larue mags are stouter and resist swelling, but have got 14 in the 20 round mag and only was able to get 10 to reliably feed. With the 300rcm being parent case of the 6.5prc 8 works better than 10 or 12 in the 20 round mags. The follower is the biggest problem in mag, larue and asc mags I have. Even trying to use 25 and 30 round mags won't help. Mags keeps you from tweaking feed lips, unless you file/sand them. I bought mags designed for the 6.5prc in hope to use the follower, but no luck.

 

[wvbearhunter]

The KAC mags work well for me when using 300wsm in my AR10. The mag allows a smidge more length and do not swell as quickly

 

[assasinator]

The KAC mags work well for me when using 300wsm in my AR10. The mag allows a smidge more length and do not swell as quickly

what model mag? ty. wow 112.00 per magazine? they are proud of a mag that barely works.....