Super low SD but crappy groups, what to do

[tylerradke]

So working on a load for my AR10 308 and I am using virgin starline brass, varget, and 175 smk with COAL of 2.810. I loaded 5 of each charge weight starting at 43grn increasing 0.3 grn at a time up to 44.5 grn. I was hoping to be 2600+fps when I found a load but my 43grn load avg just over 2500 and had an ES of 7 fps and SD of 3. Now I could deal with the slower velocity if it shot well but it shot about a 2" group at 100. Now being an AR I don't have a ton of room to play with seating depth to get it to tighten up but I will spend the time and resources doing so if someone smarter than me thinks it's worth my time and money to explore. Otherwise I'm tempted to keep creeping up charge weight until the stars align and I get the velocity I was after and a good grouping. Factory m118lr shoots just over 2600 and groups around .75-1.00 out of this rifle, and at 44.5 I am just under 2600 fps and no real pressure signs. I will go higher am just leery because 45 seems like a real hot charge out of a gas gun. Maybe I'm just being a chicken too.

 

[straightshooter1]

So working on a load for my AR10 308 and I am using virgin starline brass, varget, and 175 smk with COAL of 2.810. I loaded 5 of each charge weight starting at 43grn increasing 0.3 grn at a time up to 44.5 grn. I was hoping to be 2600+fps when I found a load but my 43grn load avg just over 2500 and had an ES of 7 fps and SD of 3. Now I could deal with the slower velocity if it shot well but it shot about a 2" group at 100. Now being an AR I don't have a ton of room to play with seating depth to get it to tighten up but I will spend the time and resources doing so if someone smarter than me thinks it's worth my time and money to explore. Otherwise I'm tempted to keep creeping up charge weight until the stars align and I get the velocity I was after and a good grouping. Factory m118lr shoots just over 2600 and groups around .75-1.00 out of this rifle, and at 44.5 I am just under 2600 fps and no real pressure signs. I will go higher am just leery because 45 seems like a real hot charge out of a gas gun. Maybe I'm just being a chicken too.

You didn't say anything about the barrel(s), like. . . what's the lengths? If you've got a 20" barrel, I think you're asking too much for a 175 SMK. . .depending on how hot your lot of Varget burns (can only tell with chrono data). With the default burn rate and a 20" barrel, here something that might give you some ideas. . .???

 

[tylerradke]

You didn't say anything about the barrel(s), like. . . what's the lengths? If you've got a 20" barrel, I think your asking too much for a 175 SMK. . .depending on how hot your lot of Varget burns (can only tell with chrono data). With the default burn rate and a 20" barrel, here something that might give you some ideas. . .???

View attachment 8472807

It is a 20" 1-10 twist

 

[918v]

Try 40.7 grains of AR Comp. I liked around 43.7 grains of Varget in FC brass in my Maten but it was too hot. 45 grains in Starline brass is too hot. You’ll wreck your gun.

 

[tylerradke]

Try 40.7 grains of AR Comp. I liked around 43.7 grains of Varget in FC brass in my Maten but it was too hot. 45 grains in Starline brass is too hot. You’ll wreck your gun.

I don't have any AR comp but I have 8208xbr which is virtually identical burn rate to AR comp. Should give similar results

 

[straightshooter1]

I don't have any AR comp but I have 8208xbr which is virtually identical burn rate to AR comp. Should give similar results

Yes. . . as much as I like AR-Comp, I think that'd be your best choice.

 

[918v]

I don't have any AR comp but I have 8208xbr which is virtually identical burn rate to AR comp. Should give similar results

No. XBR will not.

 

[tylerradke]

Would 4064 be a better option? Haven't used it before, is it temp sensitive?

 

[918v]

It’s not sensitive in your application but it’s not going to give you 2600 FPS.

 

[Doom]

20" Barrel with a 175 is going to be around 2500 fps maybe 2550 with IMR 4064.

 

[Doom]

To the OP, what version of "factory M118LR". A picture of the box and marking would help. There are numerous versions of that cartridge around and they vary in both bullet and powder.

 

[FredHammer]

OP - Is there a particular reason you are shooting 175s? Like, 1000yd shooting? If so, the magic MV to keep the 175 stable "ish" is 2550. If you aren't shooting LR, then try some 155s or 168s. Barrel just might like the different weights or bullet profiles. Good job on the ES/SD, your combustion is spot on, but the harmonics are not.

 

[ShootnFool8]

Tag...

 

[tylerradke]

To the OP, what version of "factory M118LR". A picture of the box and marking would help. There are numerous versions of that cartridge around and they vary in both bullet and powder.

It was winchester lake city 2021 manufacture brown box m118lr

 

[tylerradke]

OP - Is there a particular reason you are shooting 175s? Like, 1000yd shooting? If so, the magic MV to keep the 175 stable "ish" is 2550. If you aren't shooting LR, then try some 155s or 168s. Barrel just might like the different weights or bullet profiles. Good job on the ES/SD, your combustion is spot on, but the harmonics are not.

I am shooting out to 1000. I am trying to get to 2600 safely because that is what my factory ammo is running so I know it's possible out of this rifle.

 

[PhrogDriver]

I am shooting out to 1000. I am trying to get to 2600 safely because that is what my factory ammo is running so I know it's possible out of this rifle.

I think 2600, or very close to it, should be doable with a 20" barrel. My GAP-10 / 18" Bartlein gets 2580 with Federal GMM 175s. My handloads are matched to that ammunition, and I'm using 41.9 g of IMR 4064. Which, I believe, is the same type/amount of powder used in Mk 316, which is apparently the replacement for M118LR.

 

[918v]

When I was using IMR4064 I got 2580 but not out of a 20” barrel lol!

 

[skyrep]

I've had great success with Hodgdon H4895. Start at 39.2 and work up. I used Lake City Brass and CCI 200 Primers at 2.800 coal.

 

[Doom]

The 118LR has gone through so many iterations I'm speculating this is MK316 Mod 1. My guess its using SMP 768 which is a ball powder that was developed by St. Marks to be temperature stable. If I were looking at trying to duplicate that I would think StaBall Match is probably as close as you can get.

It might be worth bareakiing a round down to very its ball powder and checking the bullet weight. Is the case stamped Win or LC?

 

[Ledzep]

Attempting to get 2600 with a specific powder because factory ammo gets 2600 with a totally different powder sounds sort of arbitrary.

Also, you ES of 7 and SD of 3 will level out if you shoot more rounds. Within 800yd, dispersion is grossly more important than velocity spreads for hit probability, too. Personally, I'd address the gas system, barrel/upper fit, or muzzle device to wrangle in dispersion, and wouldn't worry about the velocity.

 

[OkieMike]

You're not going to safely or efficiently glean 2600 fps from a 20" gasser pushing 175's.

And it doesn't matter what the SD is if it won't group for shit.

Mike