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™, 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/showthrea...rt+barrels

[J_Von_Random]

http://www.65grendel.com/forum/showthrea...rt+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™, 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]

I would be interested to see the results. Uncharted territory for sure, but with a fast twist it may be achievable. It won’t be easy with a barrel in the 7.5-8.5” range due to movement of the weapon, but on a good solid front rest and rear bag it should be easy enough out to 400-450. Keep us posted

[J_Von_Random]

I would be interested to see the results. Uncharted territory for sure, but with a fast twist it may be achievable. It won’t be easy with a barrel in the 7.5-8.5” range due to movement of the weapon, but on a good solid front rest and rear bag it should be easy enough out to 400-450. Keep us posted

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]

For shooting at distance I think it helps to stay above the transsonic zone. Precision Firearms posted velocities from several short 6.5 Grendel barrels in Jan 2017. These velocities and a ballistic computer will give you an idea how much velocity you’ll have left at 600 yards from different barrel lengths.

Short barrels with Grendel
Hello Horde:

Just thought I would let you guys know about a test we did recently on Short Barrels. We tested:

7.5 Bartlien at 100 Yards. Groups averaged 1.5 Inches 2000 FPS 123 Match King
8.5 Bartlien at 100 Yards. Groups Averaged 1.4 Inches 2100 FPS 123 Match King
10.5 Bartlien at 100 Yards. Groups averaged .9 Inches 2200 FPS 123 Match King
12.5 Lilja Blank Chambered by PF at 100 Yards. Groups .75 2300 FPS 123 Match King
14.5 Lilja Blank Chambered by PF at 100 Yards. Groups .5 2350 FPS 123 Match King
14.5 Krieger at 100 Yards. Groups averaged .38 2375 FPS 123 MatchKing

We have most of these available for shipment with 2 weeks. They are chambered - Just need Muzzle Threaded.

We also have 7.5, 8.5, 9.5, 10.5, 12.5 in Criterion although we have not tested accuracy extensively. 12.5 we tested shot under 1 moa.

[Bigs28]

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

[J_Von_Random]

For shooting at distance I think it helps to stay above the transsonic zone. Precision Firearms posted velocities from several short 6.5 Grendel barrels in Jan 2017. These velocities and a ballistic computer will give you an idea how much velocity you’ll have left at 600 yards from different barrel lengths.

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.

[Old Bob]

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.

I used a 4x ACOG for years on my 3 gun AR. I had no problems shooting out to 400 yds with it. Passed that, hitting steel was iffy. It can be done but it might take a few shots to get there. Targets are tiny out to 500 yds or longer with a 4x scope.

[Lastrites]

Have fun with your project as it looks like it could be a blast, for me the juice isn't worth the squeeze going much further south than 11.5". I bet the fireballs off a 7.5/8.5" will be impressive, especially with a brake.

[J_Von_Random]

It's going to be a while; I already have projects that I need to finish / guns I haven't had the chance to shoot yet due to the current nonsense.

But when the time comes I will absolutely be posting the results.

[LRRPF52]

8.5" Grendel shooting the 123gr ELD-M is supersonic to 630yds at sea level/standard atmospheres if the mv is 2100fps.

You still get better ballistics than a 16.3" AK once you hit 220yds, and then surpass it from thereon out.

At 100yds, it still is really close to the AK in terms of fps and energy, assuming top mv for factory 123gr 7.62x39.

The 10.5" Grendel gives you 680yds supersonic reach with the 123gr ELD-M assuming 2200fps mv, and surpasses the 16.3" AK just after 100yds.

Switching to the 123gr SMK or Scenar will give the 10.5" Grendel an 825yd supersonic reach (using G7 drag profile, not the unrealistic G1 profile that would put it well over 900yds).

8.5" Grendel PDW
123gr ELD-M 2100fps mv

100 1926fps 1013ft-lbs
200 1759fps 845ft-lbs .8 / .6
300 1599fps 698ft-lbs 2.2 / 1
400 1446fps 571ft-lbs 3.8 / 1.3
500 1299fps 461ft-lbs 5.7 / 1.8
600 1162fps 369ft-lbs 8 / 2.2
630 1124fps 345ft-lbs 8.7 / 2.4

10.5" Grendel PDW
123gr ELD-M

100 2022fps 1116ft-lbs
200 1850fps 935ft-lbs
300 1686fps 776ft-lbs
400 1528fps 638ft-lbs 3.4 / 1.2
500 1378fps 518ft-lbs 5.1 / 1.6
600 1234fps 416ft-lbs 7.1 / 2.1
680 1129fps 348ft-lbs 9.4 / 2.6

123gr SMK or Lapua Scenar 2200fps

100yds 2049fps 1147ft-lbs 0 / .3 mils (drift)
200yds 1904fps 990ft-lbs .8 mils / .5 mils
300yds 1764fps 850ft-lbs 2 mils / .8 mils
400yds 1631fps 726ft-lbs 3.3 / 1.1
500yds 1502fps 616ft-lbs 4.9 / 1.4
600yds 1378fps 519ft-lbs 6.6 / 1.8
700yds 1261fps 434ft-lbs 8.6 / 2.1
800yds 1150fps 361ft-lbs 10.8 / 2.6

At my altitudes, the 10.5" with 123gr SMK is supersonic past 1000yds using G7 drag model, but the air is thinner up here.

[J_Von_Random]

8.5" Grendel shooting the 123gr ELD-M is supersonic to 630yds at sea level/standard atmospheres if the mv is 2100fps.

You still get better ballistics than a 16.3" AK once you hit 220yds, and then surpass it from thereon out.

At 100yds, it still is really close to the AK in terms of fps and energy, assuming top mv for factory 123gr 7.62x39.

The 10.5" Grendel gives you 680yds supersonic reach with the 123gr ELD-M assuming 2200fps mv, and surpasses the 16.3" AK just after 100yds.

Switching to the 123gr SMK or Scenar will give the 10.5" Grendel an 825yd supersonic reach (using G7 drag profile, not the unrealistic G1 profile that would put it well over 900yds).

8.5" Grendel PDW
123gr ELD-M 2100fps mv

100 1926fps 1013ft-lbs
200 1759fps 845ft-lbs .8 / .6
300 1599fps 698ft-lbs 2.2 / 1
400 1446fps 571ft-lbs 3.8 / 1.3
500 1299fps 461ft-lbs 5.7 / 1.8
600 1162fps 369ft-lbs 8 / 2.2
630 1124fps 345ft-lbs 8.7 / 2.4

10.5" Grendel PDW
123gr ELD-M

100 2022fps 1116ft-lbs
200 1850fps 935ft-lbs
300 1686fps 776ft-lbs
400 1528fps 638ft-lbs 3.4 / 1.2
500 1378fps 518ft-lbs 5.1 / 1.6
600 1234fps 416ft-lbs 7.1 / 2.1
680 1129fps 348ft-lbs 9.4 / 2.6

123gr SMK or Lapua Scenar 2200fps

100yds 2049fps 1147ft-lbs 0 / .3 mils (drift)
200yds 1904fps 990ft-lbs .8 mils / .5 mils
300yds 1764fps 850ft-lbs 2 mils / .8 mils
400yds 1631fps 726ft-lbs 3.3 / 1.1
500yds 1502fps 616ft-lbs 4.9 / 1.4
600yds 1378fps 519ft-lbs 6.6 / 1.8
700yds 1261fps 434ft-lbs 8.6 / 2.1
800yds 1150fps 361ft-lbs 10.8 / 2.6

At my altitudes, the 10.5" with 123gr SMK is supersonic past 1000yds using G7 drag model, but the air is thinner up here.

I have been Witnessed! I shall ride in Valhalla Shiny and Chrome!

ahem

I've been using the 1300fps expansion minimum of the 129gr ABLR for my back of the envelope effective range figures. Whichever ammo is used 500yds effective range appears entirely doable from an 8.5". Possibly even from 7.5, but a little extra margin would be preferable.