I was going to order the kit from Sig but not sure which one to get, 6.75" or 9". Does anyone know what the velocity is for Sig 220 grain ammo out of both barrel lengths? I called Sig and they didn't know.

It's hard to say for certain, and I doubt anyone on the webz has exact data. But ballparking from what is available on the webz, you might get 950-975 fps from the 9" barrel. Maybe a loss of 60-100 fps going down to the 6.75" barrel. Maybe an even greater MV loss.

Run the ballistics for a wild ass guess on kinetic energy and flight curves. And consider how the bullet performs at various velocities.

Are you going to shoot suppressed? If you are what is the minimum length allowed by the suppressor manufacture.

Can issues aside, I would go with the 9" I have an 8" sbr and the the Ricky Bobby statement, "I am not sure what I am suppose to do with my hands." Comes into play with the 7" rail. With a shorter rail it would be worse.

Or is this for CQB in boats?

I'm using Sig's 762SDTi can. I shot a 6.75" at Sig's range day at the SHOT show but the guy there said they were specifically for a certain Spec ops group and I thought that meant never available to us. Now they are and I don't have enough info to buy

I stick with the 9" barrel, as the round was perfected around that length. It's a better balance for both subs & supers...

quote:

Originally posted by CandyMan.45:
I stick with the 9" barrel, as the round was perfected around that length. It's a better balance for both subs & supers...

Agreed. My understanding is that the 300 blk was designed to burn all of its powder in a 9" barrel.

Anything longer or shorter than that and you're losing velocity.

quote:

Originally posted by esdunbar:

quote:

Originally posted by CandyMan.45:
I stick with the 9" barrel, as the round was perfected around that length. It's a better balance for both subs & supers...

Agreed. My understanding is that the 300 blk was designed to burn all of its powder in a 9" barrel.

Anything longer or shorter than that and you're losing velocity.

Noooooo. You still gain velocity post burn as the gases are still pushing the bullet. 16" hits with a lot more velocity than a 9".

quote:

Originally posted by guardianangel762:

quote:

Originally posted by esdunbar:

quote:

Originally posted by CandyMan.45:
I stick with the 9" barrel, as the round was perfected around that length. It's a better balance for both subs & supers...

Agreed. My understanding is that the 300 blk was designed to burn all of its powder in a 9" barrel.

Anything longer or shorter than that and you're losing velocity.

Noooooo. You still gain velocity post burn as the gases are still pushing the bullet. 16" hits with a lot more velocity than a 9".

Yes, you're right, derp moment for me there.

I'm not sure where friction overtakes was pressure, but is certainly not just beyond 9"..and not at 16 probably either. Thanks for correcting that.

Still want to make sure it's at least 9" or else he'd be sacrificing velocity though...don't want my brain fart to distract from his question so that's my attempt to re-rail it.

quote:

Originally posted by guardianangel762:
Noooooo. You still gain velocity post burn as the gases are still pushing the bullet. 16" hits with a lot more velocity than a 9".

With supersonic ammo you are absolutely correct. As long as the pressure of the burnt gases is greater than the friction of the bullet in the bore, bullet velocity continues to increase. A 16" barrel provides a few hundred (maybe many hundred) more fps velocity than a 9" -- enough to noticeably improve flight ballistics and kinetic energy. As barrel lengths increase, the relative pressure on the bullet decreases, which is why each 1" increase in barrel length produces a progressively smaller increase in bullet velocity.

However with heavy subsonic 300 blk bullets, the difference in MV between 9" and 16" barrel lengths isn't all that much. It's my understanding that the 16" barrel generally will increase the MV a little closer to Mach 1, but still be subsonic on a properly developed load. This is because subsonic loads don't have much powder in the case, and thus the chamber pressure is pretty low to start with.

*****
There isn't any magic in the 300 blk's design to have such MV characteristics. It's a combination of bore size, powder capacity, and bullet weight. Nobody designs a cartridge around a barrel length. Between prior experience and computer modeling (think fluid dynamics), once the 300 blk case size was ballparked, the cartridge's ballistics were pretty much locked in.