Seating depth and pressure question

[oneofthose]

I cast some bullets with a lee C309-120-R with gc, sized to .308. The load data calls for 1.680 COL, but they won’t easily chamber in the ol’ .30 carbine at that length. I had the slap the bolt closed with my palm to get them to chamber. I fired 2 rounds that way before realizing the problem. They registered 1600 and 1608 fps, with 10.0 gr H-110 (starting load).

At 1.605 (.075 shorter) they will chamber and leave just the slightest indication of touching the rifling, but is there a potential pressure problem at this length?

Data for the 110 gr fmj’s I’ve used in the past also calls for a 1.680 seating depth, which works just fine. The problem is the ogive of the cast bullet; it’s fatter at the nose and touches the rifling before the bolt locks.

[williamwaco]

I cast some bullets with a lee C309-120-R with gc, sized to .308. The load data calls for 1.680 COL, but they won’t easily chamber in the ol’ .30 carbine at that length. I had the slap the bolt closed with my palm to get them to chamber. I fired 2 rounds that way before realizing the problem. They registered 1600 and 1608 fps, with 10.0 gr H-110 (starting load).

At 1.605 (.075 shorter) they will chamber and leave just the slightest indication of touching the rifling, but is there a potential pressure problem at this length?

Data for the 110 gr fmj’s I’ve used in the past also calls for a 1.680 seating depth, which works just fine. The problem is the ogive of the cast bullet; it’s fatter at the nose and touches the rifling before the bolt locks.

Very simple concept:

If it works - use it.

If the bolt will not close the stated OAL is not correct for that combination.

The purpose of the industry standard OAL is to insure that ammo manufacturers do not make ammo too long to work reliably. Assuming the the weapons manufacturers insure their equipment will work with the standard length ammo. - It is a vicious circle.

Fortunately, in this case you have total control of the entire situation. You are in control lf the gun and the ammo. Make them work together. Select a length that works reliably then if you are worried about the pressure, reduce the .30 carbine load a half grain and test it out.

[geargnasher]

Yes, seating deeper will increase the pressure, all else being the same. However, you're using starting loads already, so I wouldn't be too worried about it even in .30 Carbine. Like WW said, cut it half a grain and try it out. You don't want to reduce 296 below starting loads in most instances if you're following the recipe closely with boolit/bullet type, seating depth, and primers.

Gear

[flounderman]

seating a bullet into the rifling will raise pressures. different chronograph readings. ideally the bullet should just about touch the rifling. you don't want too much clearance and you don't want it jammed into the rifling.

[zomby woof]

I believe my OAL for that boolit in 30 Carbine is 1.610. You're fine.
I size mine to .310" and use 12.5 grains 296/H110

[oneofthose]

Thanks for your replies. I cranked my remaining rounds down to 1.605 last night and fired them today. They all cycled flawlessly.

The 10 gr loads averaged 1503 fps, and none were near the 1600+ that resulted with the two that were jammed into the rifling with the longer oal, just as flounderman suggested above. My original concern was that the reduced case volume would increase pressures, but it looks like that was more than offset by backing the bullet out of the rifling.

10.5 gr loads averaged 1552 fps.
11.0 gr loads averaged 1580 fps.

I'll put together some more and see how they group.