Getting Started with 7" Barrel Suppressed AR15 Pistol

[flamatrix99]

Since you will lose velocity in a 7" barrel, I would start around 10.5-11 grains, with the bullet seated long, around 2.250"

Funny you mention these numbers. I loaded 5 rounds at 2.250" with 10.5 gr of A1680. I was also going to do 10.3 and 10.7 gr. Then chrono them this weekend.

[alamo5000]

Holy crap, I never even thought seating depth played a role in tuning.

It's probably one of the more important parts actually regardless of caliber.

The seating depth is one of the big three.

Case weight (or just plain old quality of cases)
Charge weight (of whatever powder)
Seating depth

If you can control those three things generally you are going to get substantially better performance out of any ammo in pretty much any gun.

You have other factors too but to me they are a bit more secondary (depending on how you look at it). Don't over crimp! Really crimping down hard depending on the type of bullet used can cause pressure to spike up big time and that's not good. That's more of a safety thing but it can impact accuracy. Crimp enough to keep your bullets where they need to be and that's it. Some calibers I don't even crimp at all.

Doing extra work like chamfering flash holes and all that might help in some calibers but as a general rule it's down the list quite far for me.

[rebel]

It's all about pressure gentleman. To add to dellet's comment, seat it longer, closer to lands - more pressure, i.e. more velocity. Seat it deeper, without compressing powder means less pressure, compression with certain powders ends up with radical increases in pressure. H110/ W296 is one of these.
The VV powders are much more forgiving for subs. Before you go hunting, test your bullet choice for expansion. Make sure it works, the 208 will not expand or do anything much but make a hole at sub velocities.

[flamatrix99]

I saw the max COAL is 2.260”. Most subsonic loads I see are around 2.1-2.25”. Is there a sweet spot for 208 gr Hornady HPBT projectiles? I am not sure where to start. This is for a PSA 8.5” AR Pistol upper.

I know ideally I need a Hornady gauge to measure my chamber but this mainly a range fun gun.

2.260” is a default to max SAAMI length or what will fit in an AR magazine. You have to pay attention when that is the given length because not all bullets can be loaded that long without hitting the lands, but the 208 Hornady is not one.

The 208 HPBT is reasonably jump tolerant so the sweet spot will likely be pretty wide. What will be more important is case fill ratio. Depending on the powder you choose, and how close to the bullet is seated to the powder for good ignition and burn will be first, then fine tune for accuracy and velocity by changing the depth.

I am using A1680. I loaded 5 rounds each with 10.3, 10.5 and 10.7 at a nominal 2.250". I am loading on a Dillon 550b so my COL seems to vary +/- .004". I will chrono them in the next couple days and see how I did.