Got My Subsonic Rounds to Cycle !!

[Pnutz]

Figured I would Share my success in getting my subsonic rounds fire and cycle out of my 16" AR15.

Specs:
Barrel 16"
Carbine Gas Port - Std Size .075?
Rifle Length Stock std buffer weight and spring

Could not find any AA 1680 as highly recomended but did find some IMR 4198 Which is very close in burn rate as listed here http://www.reloadbench.com/burn.html Loaded up A Hornady 168gr Match round with 10.3grs of IMR 4198. All 5 loaded rounds cycled perfectly. Thx for everyones input thought I would share mine.

[Fudmottin]

Nice. Have you run any over a chronograph yet?

My research says Mach 0.9 is when the bullet flight noise starts getting loud. This speed can be as low as 1089fps / 332 m/s. You will see slightly higher numbers posted. The speed is a function of air density in general.

When I say my research, I mean reading reported findings of other people, including Hiram Maxim.

[avnut]

I tried this load using what components I had.I used 10.3 gr H4198 with a Remington 165 gr bullet.They all shot well and cycled the action every time.
I have a M1S 16" carbine port location.Load seems very mild.

[Pnutz]

When using 180g Bullets the 10.3g of 4198 Isn't enough to reliably cycle the action. I stepped it up to 11.5g but kinda think I might be going supersonic or transonic. I'm gonna try lowering it a bit to see what I can come up with. Maybe try 11g of 4198.

[Rikky Lee]

Speed of sound is a function of air temperature. The hotter the ambient temperature the higher the speed of sound. Use this link to build your own table and see what happens in your area.

http://www.srh.noaa.gov/elp/wxcalc/speedofsound.shtml

Remember successful use of subsonic projectiles comes at the heavier end of the caliber. So for 30 cal start looking at 200 grain bullets as a minimum but true success (ballistic happiness?) really happens at 220 to 240 grains.

But you have to have a 1/8 twist barrel for accuracy.

[Pnutz]

Thx for the Data Ricky, I'm unsure of the twist in my barrel but is seems to stabalize 180 and 208 grain bullets really well. I tried some 150g supersonic but the groups at 50 yards sucked.

Just got back from Hunting Sage rats and had no trouble picking them off at 150yards with 180g Bullets and 11.5g of IMR 4198

[Rikky Lee]

Good results - glad to help.

You know how to check your barrel twist?

Put a path on a solid cleaning rod into the chamber - mark the rod at the top with a marker pen near your hand - in line with the stock or something easily checked by you - doesn't have to be exact.

Or you can pull the rod through the muzzle taking the usual precautions and mark where the rod touches the action.

Push the rod along the barrel until the mark on the cleaning rod has gone a full circle. Mark the rod again.

Remove and measure. There is your indicative barrel twist.

Try it - if the heavies stabilize you may well have the 1/8 twist.

Also - a fast twist will not degrade lighter bullet accuracy - a bit more powder helps. I had the same issue building a 155 grain load in mine.