loading for specific twist rate

It may just be my imagination but lately the Chrony seems to be telling me that my 12" twist .223 prefers a velocity of around 3220 fps or so, irrespective of bullet weight. It first became noticeable with 64 grain Bergers but now that I'm looking even the 55's have been dropping hints.

3220 fps seems slow for a 30" barrel, especially for 55 grainers: is barrel length being trumped by rate of twist? If so, does this mean I should I stay at/under 3200 fps and use the propellant that gives best pressure for my favorite projectiles, at that velocity?

I'm still scratching my head over this. I mean, look at the data for .22-250, for example: the 14" twist test barrel is giving much higher velocity for 55 grain bullets, using the same pressure! Thanks to anyone who can shed some light on this phenomenon.

murf said:

more powder, less pressure for a given velocity...

Click to expand...

...thanks murf, this is something worth chewing on. I'm trying my best to comprehend some basics here, even after years of reloading. Some of this stuff I've known about, like the fact that an expanding powder plasma can move a projectile more efficiently and with less heat than expanding gases, which are more elastic. Maybe I'm dealing with harmonics? All I know is that the 64's bounced back and forth within a very tight range, 3210-3220 fps and I would be a happy camper if I could get the 55's to do the same.

Those 64's were charged with varget, which admittedly seems to burn a little cleaner than other propellants I've tried. Since there does seem to be a relationship between the level of fouling and SD, maybe the key here is to find a propellant that will give the least fouling with the 55's, at 3220 fps. And since I know that fouling often accompanies low pressure... I'm looking at the data and I see two propellants, IMR 4895 and H335 which max out at around 3220 with the 55's, they might be worth a shot - the H335 uses less actual propellant - but I've already got H4895 and BL-(C)2 in the bin and they're fairly similar so I think I should finish testing those first. Maybe load for a slightly lower velocity to allow for deviation due to fouling? My present goal is pretty modest: all I want are the tightest five-shot groups I can get with the 55's between swabbings.

Anyway thanks for straightening out the confusion about powder charge and velocity. Makes sense now that I think about it. Also makes sense that the additional powder charge would contribute to fouling as well as velocity!

For me velocity itself obviously is only as important as it relates to accuracy. Normally slower shots land higher (a phenomenon that really blew my mind until it was explained by Bart B.), but under certain circumstances it doesn't happen. Okay I'll try to explain:

Here is a group from this morning's shoot at 200, 55 gr Nosler BT's, Nosler cases, 26.1 grains BL-(C)C2, CCI #450 primer;

Velocity:
3278
3332
3329
3339
3296

I attempted to load for a starting velocity of 3200 fps but as you can see it got away from me. The fouling shot is the slowest and landed highest. The rest of the group is tight, but the velocity of the last round is already starting to slow, presumably due to fouling. For five simple shots and I've got a spread of 61 fps. I'm thinking if I have a slightly lighter charge (3200 fps?) that doesn't drag against the bore so much the fouling shot won't be set apart like it is and the fifth shot won't slow so much.

The question, it seems to me, is at what velocity does centrifugal force itself assure good contact with a 12" twist bore? Surely this is known?

post script: after thinking this through I'm not so sure that's the fouling shoat at the top. I may in fact have to increase the charge somewhat. This is what's so confusing about velocity and elevation...