Velocity inconsistancy

[Medic650]

I'm having trouble getting my velocity to be consistant. I loaded 50 rds today with 168gr bthp on top of 1680 powder. I checked several rounds and was within a 1/10th gain on my powder charge. I had up 347fps extreme spread. I'm using a Dillon 550 and Reminton 223 brass. I'm loading for subsonic. Do I need to trickle charge my powder for each round?

[Fudmottin]

It doesn't sound like your problem is with powder charge consistency. Look for other problems such as neck tightness, seating depth, etc. Unless you are running a really old batch of powder or primers, that part should be kosher.

You could also check that your brass all has the same internal volume. I expect this is less important than neck tightness and seating depth.

[Medic650]

Primers I've had for a few years but powder is new. The brass is all new formed from WW 223 brass. I'm using a SSK upper and I'm seating just deep enough to fit in a AR mag. My SSK upper will not chamber LC or other military brass. With the 168 BTHPs I'm wondering if I'm not getting enough powder in the case and the shorter bullet, vs a 220 SMK, that the powder has some void to change position. I loaded another 25 last night and weighed every charge and was always within 1/10th of a grain out of the powder measure. I adjusted charge so I'll see if that makes any difference.

[Rikky Lee]

Velocity behavior like that indicates a need for faster powder to give more consistent internal combustion from shot to shot. This leads to more consistent chamber pressures etc.

So what? I have moved two powders up the burning rate chart and am now in 9mm pistol territory for burn rate. Fills the case better in low volume loads.

[Medic650]

I forgot to mention my gas port is at pistol position so I might need to try a faster powder. The 168gr bullets were some of Wideners mil-spec bullets. I've found there is a few grains weight difference and overall length difference also. Those two things my explain my problem.

[LouBoyd]

Originally Posted by Medic650

I'm having trouble getting my velocity to be consistant. I loaded 50 rds today with 168gr bthp on top of 1680 powder. I checked several rounds and was within a 1/10th gain on my powder charge. I had up 347fps extreme spread. I'm using a Dillon 550 and Reminton 223 brass. I'm loading for subsonic. Do I need to trickle charge my powder for each round?

There is a possibilty you may be overlooking completely. I had an extreme case of velocity variation with a 300 whisper I was experimenting with. What calcultated to be a 900 fps load when fired left the bullet still sitting in the chamber though there was a loud report as the propellant went out of the barrrel. That firearm was a TC Contender using 221 brass and if memory serves me the bullets were 168 SMKs. I do remember though they were about 0.1" off of the lands. I don't know if this is your problem but it's worth checking.

If the bullet is seated too far back in the case, a large amount of the propellant gas can go around the bullet and out the barrel before the bullet moves forward far enough to seal against the lands. My suggestion is to move the bullet forward far enough to just seat on the lands. Depending on your chamber and the bullet shape you use that may not be practical. If the bullet won't reach the lands choose a different bullet which will touch the lands. Yes touching the lands and/or using a heavier bullet will produce higher pressure so work up a new load carefully.

A few tenths of a grain in powder weight nor a couple of grain in bullet weight cannot account for a 35% velocity variation. A 35% drop in velocity requires about a 50% loss of propellant energy being transferred to the bullet.

[Medic650]

Originally Posted by LouBoyd

There is a possibilty you may be overlooking completely. I had an extreme case of velocity variation with a 300 whisper I was experimenting with. What calcultated to be a 900 fps load when fired left the bullet still sitting in the chamber though there was a loud report as the propellant went out of the barrrel. That firearm was a TC Contender using 221 brass and if memory serves me the bullets were 168 SMKs. I do remember though they were about 0.1" off of the lands. I don't know if this is your problem but it's worth checking.

If the bullet is seated too far back in the case, a large amount of the propellant gas can go around the bullet and out the barrel before the bullet moves forward far enough to seal against the lands. My suggestion is to move the bullet forward far enough to just seat on the lands. Depending on your chamber and the bullet shape you use that may not be practical. If the bullet won't reach the lands choose a different bullet which will touch the lands. Yes touching the lands and/or using a heavier bullet will produce higher pressure so work up a new load carefully.

A few tenths of a grain in powder weight nor a couple of grain in bullet weight cannot account for a 35% velocity variation. A 35% drop in velocity requires about a 50% loss of propellant energy being transferred to the bullet.

I'm shooting an SSK upper on an AR15. I am seating bullets as far out as I can and still fit in a magazine. I guess something else I could try is putting little more crimp pressure on. The bullets I'm trying right now don't have a cannalure but, I could have bullets being pushed back into the case a little. I've never had that problem, at least with 223, before though.

[Fudmottin]

That's a new one to me too.

I know the bench guys like to use as little crimp as possible (IIRC). But with an autoloader, you should have some because the bullet can get pushed back a bit when chambering.

I've had a couple bad magazines where the nose would hit right below the feed ramp and the spring energy would push the bullet over half its length into the case. That was with the crimped 55gr bullets with canalur.