.308 175gr SMK 16" Barrel.. Powder?

[griff383]

I have an AR in .308 with a 16" barrel and started thinking about what powder might best suit this set-up. I currently use Varget for 308 but I have Benchmark, TAC, BL-C(2), IMR 4895, and IMR 4064. Would a faster powder (than Varget) be better suited to my particular rifle?

 

[Zak Smith]

It won't give you more velocity. All you can get is less flash at the expense of less velocity.

 

[MEHavey]

At 55,000 psi, all these will fill the case 95% - 105% and burn 96-100% in that 16" barrel:

IMR3031 Best Velocity
IMR4895 Second Best
RL-15 Third Best
A4064
A2520
RL-17
IMR4064
H4895
AA2495
N540
N130
N135
N133
VARGET

 

[REL1203]

i shoot 175 SMKs out of a 18" barrel with RL15 and have great accuracy, i bet it would work just as well in 16"

 

[griff383]

Thanks for the info, is one (or more) of those powders less sensitive to temperature changes? I hunt in Minnesota and the tepmerature can range from 0 to 70 during deer season

 

[helotaxi]

You'll also find that those same powders will perform well in a 14-26" barrel.

Chamber pressure is your limiting factor on powder speed, not barrel length. To use a faster powder, you have to reduce the charge density to keep the pressure from blowing the rifle apart. What you also end up with is a pressure curve with a sharp spike at ignition that quickly drops to a very low pressure. The fast powder creates a high initial impulse but cannot sustain the pressure as the volume behind the bullet expands because 1) the powder all burned almost immediately to create the initial spike and 2) the charge was reduced meaning that it was only able to create a small amount gas to begin with.

From the perspective of complete burn and velocity, 3031 is about as good as it gets in the medium weight bullets in .308. Most of the other suitable powders will have burn percentages in the mid to high 90% range over any practical barrel length as well with a variance of less than 5% over a 12" range of barrel length.

 

[popper]

MEHavey -- Does H335 or 8205 XBR work in 22" AR .308? Both seem to be close to 4895.
Others seem to like varget, but isn't it really slow?

 

[Zak Smith]

Varget is my go-to powder in .308.

It is not "really slow" compared to 4895-- it is a little slower. Most burn rate charts have them very close together. In the scheme of things they are very close.

748 -> BLC2 -> H335 -> IMR 4895 -> N135 -> Varget -> IMR 4064 -> N140 -> RL15

Even with the above order (fast to slow per reloadbench.com), I get more velocity with less pressure signs with Varget vs. N140.

In my experience, Varget is very temperature insensitive.

 

[griff383]

Varget has been good to me too, just making sure I couldnt be doing something better.

 

[MEHavey]

168SMK and 22"barrel at max practical working pressure...

"The Standard"
IMR 4895
Weight: 44.2gr
Casefill 98.4%
Velocity: 2671fps
Burned: 98.5%
Pressure 54,000psi

Compare....
Hodgdon H335
Weight: 42.5gr
Casefill: 86.7
Velocity: 2621fps
Burned 100%
Pressure: 54,000psi

BOTTOM LINE: H335 will work, but 50fps slower; Lower case-fill using a ball powder will likely result in higher velocity variances. (Theory only, of course)

`Didn't find XBR in the lineup, so I'll call the QL people.