Longshot in longer barrels

[Pablo]

I loaded some HOT 180gr rounds with Longshot last night. I did a series of rounds from 9.7gr to 10 gr for my AR.  Now I'm getting cold feet. I am going to walk in slowly and watch for pressure signs. Heck, I'm gonna chute it. The AR chamber is super supported and thick.

Anyway, I keep reading Longshot looses it's FPS mojo at 9.2 to 9.4 grains. This is because? Will barrel length follow (support) more powder charge, and commensurate higher velocity?

[The_Shadow]

Pablo, I think what people were seeing in the handguns with the LongShot was the recoil systems starting being overcome and thus the velocity was not showing gains for them.  Yes chamber support is also a major player as you approach the SwampFox levels...

[DM1906]

9.7-10 gr. Longshot, while over published max, isn't scary. I've gone well past that with no issues, other than the velocity plateau in shorter barrels. In 6.5" barrels, it uses that extra powder and really gets a boost. Just for reference, 9.5 gr. Longshot with a 180 gr. JHP at 1.260" is only 34,600 CUP, well below the ceiling. Exceeding that didn't seem to ramp pressures to any exceptional degree, and shown no greater pressure indicators beyond 10 gr. Don't sweat it in your AR, but I wouldn't light it off in a Delta Elite or Gen 2 Glock.

[Pablo]

My barrel is ~11"

[Benchrst]

Curious about your results. In my 5.15" barrel Longshot ran out of steam between 8.8 & 9.4gr, depending on the 180gr bullet.

[The_Shadow]

At 9.4 grains LS under a 180 you should be seeing 1250 to 1260 fps from a 5" BBL...are you using a mag primer?
I work up using the CCI350's as some powders like LS are dense and the extra bisance of the magnum primer helps with a more uniform ignition.

[Benchrst]

5.15" QL predicts 1,281 with 9.3gr behind a 180 MG - I got 1,273 at 37deg.

Same load out of an 11" QL predicts 1,444.

That's 833 ft lbs of 'go away'

[Benchrst]

I work up using the CCI350's as some powders like LS are dense and the extra bisance of the magnum primer helps with a more uniform ignition.

Identical load (8.9gr 165 MG) / conditions I found an 84fps increase with mag primers.

[Pablo]

5.15" QL predicts 1,281 with 9.3gr behind a 180 MG - I got 1,273 at 37deg.

Same load out of an 11" QL predicts 1,444.

That's 833 ft lbs of 'go away'

Wow. Thanks. What about 9.6gr and 10 gr?

[The_Shadow]

Benchrst, what is the QL predicted pressures for the 9.4 grain LongShot load with a CCI 350 with a 180XTP COAL 1.2520"

One QL graph showed LS 9.4 with 200gr XTP Starline Brass at 58,823 PSI COAL 1.2500" 5.00" BBL @ 1293 FPS
Real world only shows 1250-1270 fps.

The program showed 24.60 grains water capacity for the Starline, in my measurements I saw:

Case capacity 10mm (water weight in grains)

Double Tap 10mm length 0.9880" weight of case un-sized with spent primer    75.0 grains

With water un-sized weight with water 99.8 grains            diff   24.8 grains
With water re-sized weight with water 99.2 grains (case mouth sized)   diff   24.2 grains

Cor-Bon 10mm length 0.9880" weight of case un-sized with spent primer    74.6 grains

With water un-sized weight with water 99.2 grains            diff   24.6 grains
With water re-sized weight with water 98.8 grains (case mouth sized)   diff   24.2 grains

JAG 10mm length 0.9805" weight of case un-sized with spent primer       74.8 grains

With water un-sized weight with water 99.4 grains            diff   24.6 grains
With water re-sized weight with water 99.2 grains (case mouth sized)   diff   24.4 grains

Star Line 10mm length 0.9855" weight of case un-sized with spent primer    77.4 grains

With water un-sized weight with water 102.2 grains            diff   24.8 grains
With water re-sized weight with water 101.4 grains (case mouth sized)   diff   24.0 grains

CCI Aluminum 10mm length 0.9875" weight of case un-sized with spent primer    27.2 grains

With water un-sized weight with water 51.8 grains            diff   24.6 grains
With water re-sized weight with water 51.2 grains (case mouth sized)   diff   24.0 grains

With water un-sized weight with water adv               24.7 grains / 24.68 grains (CCI added)
With water re-sized weight with water adv               24.2 grains / 24.16 grains (CCI added)

[Benchrst]

If QL has the ability to manipulate the primer function I'm not aware of it - the software's model for ignition is a one size fits all.

Where the program really shines is bottle necked cartridges, straight wall is hit or miss.