PDMR, or; How I learned to stop worrying and love the short barrels.

[J_Von_Random]

Over the last few days I've been reading a lot about the grendel's amazing performance in SBRs/pistols. I was already interested in building a long grendel Someday(tm), particularly after getting one of the PSA VCDL lowers which seemed a good match given this country's long standing culture of marksmanship. Now I'm interested in an extremely short build. I know the optimal short barrel length is around 12", but my goal here is more a proof of concept rather than optimization.

What I'm looking for is something that can get solid hits out to DMR range (400-600yd, 600 is the longest I have access to) with commercial ammo. Switching to higher end commercial (like the nosler 129gr) would be acceptable at the far end of the range, but most of the range should be accessible with "normal" ammunition. I'm not a hunter, so in practice this would be a target shooting build. I don't have a completely clear idea of what is good enough to count as a "solid hit"; that is one of the questions.

The tricky part is that I want it to be the shortest possible barrel that can achieve this. Starting at 7.5", willing to go a bit higher, but do not want to go much higher if at all possible, even if that means trimming some of the range.

The idea of an AR short enough to be a PDW, that can also work as a near-DMR has thoroughly tickled my fancy.

(they said I was mad, Mad! Well I'll show them all!)

[Bigs28]

First question. What elevation are you shooting at?

[Bigs28]

http://www.65grendel.com/forum/showthread.php?13307-Short-barrels-with-Grendel&highlight=Short+barrels

[J_Von_Random]

http://www.65grendel.com/forum/showt...=Short+barrels

Read that, is the first place this idea started to form.

First question. What elevation are you shooting at?

Around here everything is ~600 feet. Close enough to sea level to not matter...

[A5BLASTER]

Over the last few days I've been reading a lot about the grendel's amazing performance in SBRs/pistols. I was already interested in building a long grendel Someday(tm), particularly after getting one of the PSA VCDL lowers which seemed a good match given this country's long standing culture of marksmanship. Now I'm interested in an extremely short build. I know the optimal short barrel length is around 12", but my goal here is more a proof of concept rather than optimization.

What I'm looking for is something that can get solid hits out to DMR range (400-600yd, 600 is the longest I have access to) with commercial ammo. Switching to higher end commercial (like the nosler 129gr) would be acceptable at the far end of the range, but most of the range should be accessible with "normal" ammunition. I'm not a hunter, so in practice this would be a target shooting build. I don't have a completely clear idea of what is good enough to count as a "solid hit"; that is one of the questions.

The tricky part is that I want it to be the shortest possible barrel that can achieve this. Starting at 7.5", willing to go a bit higher, but do not want to go much higher if at all possible, even if that means trimming some of the range.

The idea of an AR short enough to be a PDW, that can also work as a near-DMR has thoroughly tickled my fancy.

(they said I was mad, Mad! Well I'll show them all!)

Go with a 12 inch barrel at min.

Sbr stamp the lower if that is legal in your state, that way you can add a heavy stock too hold down the weapon.

Use a heavy handguard as well

Put a brake on it and use adjustable gas on a carbine length system, add in a lowmass bcg and lowmass buffer/spring system.

Drop on a scope with atleast 20 power on the max end, fill the mag with 123 eld-ms and have fun with it.

I have taken my 12 and 12.5 too 600 on steel plates, it's easly done.

But if I was going too make a dictated rig on a short barrel, what I posted above is what I would go with.

Hope this helps sir.

[J_Von_Random]

I have taken my 12 and 12.5 too 600 on steel plates, it's easly done.

But if I was going too make a dictated rig on a short barrel, what I posted above is what I would go with.

Hope this helps sir.

Sadly no. I know the 12" can easily reach 600, and with good ammunition reach 1000. I'm curious about the shortest possible rifle that can reach 600 (or slightly less).

[J_Von_Random]

Ok, I played with the ballistics calculator Hornady has on their site some more than I had previously.

Given the values from the PF short barrel test, and what appears to be the accepted wisdom that 123gr out of a 12.5" is fine for 600 yards, a 7.5" barrel would be good for about 450 yards. 8.5" gets it up to 500.

Guess I'd better start saving up

[ricsmall]

[J_Von_Random]

Yep, "uncharted territory" confirms what I thought from the lack of information coming up in searches. Once I do it I will definitely post results together with crow eating / brags as appropriate. Though it will be some time before I'm in a position to start this build.

Which leads me to my next question: If it wasn't already obvious I am new to long range shooting. I keep hearing wildly differing advice for DMR scope magnification at these ranges: some say that a 6 or 8x works, some that you need 15-20x. I lean heavily towards 1-8 LPVO if possible, based on the jack of all trades nature of this build idea.

[A5BLASTER]

Yep, "uncharted territory" confirms what I thought from the lack of information coming up in searches. Once I do it I will definitely post results together with crow eating / brags as appropriate. Though it will be some time before I'm in a position to start this build.

Which leads me to my next question: If it wasn't already obvious I am new to long range shooting. I keep hearing wildly differing advice for DMR scope magnification at these ranges: some say that a 6 or 8x works, some that you need 15-20x. I lean heavily towards 1-8 LPVO if possible, based on the jack of all trades nature of this build idea.

If all your going to do is bang steel a 1-6 or 1-8 is plenty. If you want to try and see how small a group it will print on paper
your going to need more glass. Something in the 20 are more range on the max side.

The heavier you can build it up the better it will handle during the shot.

[VASCAR2]

[Bigs28]

I bang steel with 1-8x out to 500 yards np with my 12 inch and American gunner ammo.

[J_Von_Random]

The Hornady calculator gives these values for 123gr ELD-M:

2000fps (7.5"): 400yd: 1359fps, 500yd: 1217fps, 600yd: 1092fps
2100fps (8.5"): 400yd: 1444fps, 500yd: 1297fps, 600yd: 1159fps

[grayfox]

Yep, "uncharted territory" confirms what I thought from the lack of information coming up in searches. Once I do it I will definitely post results together with crow eating / brags as appropriate. Though it will be some time before I'm in a position to start this build.

Which leads me to my next question: If it wasn't already obvious I am new to long range shooting. I keep hearing wildly differing advice for DMR scope magnification at these ranges: some say that a 6 or 8x works, some that you need 15-20x. I lean heavily towards 1-8 LPVO if possible, based on the jack of all trades nature of this build idea.

How much magnification and glass clarity depends heavily on the status of your eyes, no one else's. What someone uses may or may not work for you. Some people are fine with a 1-8, others need 14 or 16x, to see well out to 600 yds.
So you should really look through lots of glass, from several vendors, before making up your mind. Me personally, I'd use a 16x to get out there to 600. But I would also use a longer pipe... so, watching this to see how you do. Go for it!

[J_Von_Random]

How much magnification and glass clarity depends heavily on the status of your eyes, no one else's. What someone uses may or may not work for you. Some people are fine with a 1-8, others need 14 or 16x, to see well out to 600 yds.
So you should really look through lots of glass, from several vendors, before making up your mind. Me personally, I'd use a 16x to get out there to 600. But I would also use a longer pipe... so, watching this to see how you do. Go for it!

Alas! It is harder to try out glass than guns. And even if glass rental were a thing there is a dearth of larger shops / ranges in my area. I want to try an ACOG as well for other guns, with the same problems.

I'm figuring that in the worst case I have a nice M4-ranged gun, in a smaller package that hits harder (and have reason to go for the next upper). Best case I discover a corner of design space that others might find useful and may not have otherwise looked at. There is no lose condition here.