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I would test a few factory rounds on the same chrono see if that tells you anything.

I found quite some variation in my 7.5x55 when seating the bullets a little deeper....with same load as before, the rifle was shooting 9" higher! Checked the scope, no problem...assume pressure increase from deeper seating....quite a difference. Perfectly good groups...just 9" higher. Surprised me.

Mike95

Just out of interest I ran your data through QuickLoad and it gave the following results:

23.6gn - 2769fps

24.2gn - 2840fps (chamber pressure = 50538psi)

Thanks Racalman... I'm getting about 100fps less than those..

Daryll.

There are two main causes of lower pressures and MVs in factory rifles against factory ammunition nominal ballistics or handloads in manuals or manufacturers' data.

One is internal barrel dimensions as previous posts say. Ammunition and powder companies use test barrels from the better custom barrelmakers and specify true SAAMI or CIP internal dimensions. Factory rifles usually have somewhat 'slacker' bores. The other and often overlooked factor is 'freebore', ie how much jump the bullet makes before hitting the lands. Again the chambering reamers used in the test barrels are specified to have SAAMI / CIP freebores .... and this is frequently much shorter than that in factory rifle chambers. Remington 700s have become notorious for huge freebores - an anti-litigation measure - and American match shooters regularly comment that new off the shelf 308 Win rifles from this company come nicely set up to load heavy match bullets in the 185-210gn classes at COALs way above standard SAAMI values. A lot of extra freebore drops pressures and MVs considerably.

If there is say 100 thou' extra freebore (and a 5.56mm chamber for the current SS109 pattern milspec cartridge is specified with much more FB than a 223 to SAAMI spec in any event) taking the 223's 'effective COAL' to 2.36", QuickLOAD estimates 24.3gn TAC's results reduce by a little over 4,000 psi PMax and 50 fps MV due to the volume of the initial combusion chamber being increased. In practice, bullets taking a large jump see even larger reductions due to their being able to 'take a run' at the rifling and being engraved in the rifling with less of a check than those with ogives almost on the lands at the time of ignition.

QL also points up another factor at play in that it calculates the charge burn of your load at a poor 91% suggesting TAC isn't an efficient powder in it. The faster burning Ramshot X-Terminator gives considerably better results in the model. (and may or may not do so in real life too.) With your combination almost certainly running at peak pressures below 50,000 psi, this is much lower than ideal for a ball type powder such as TAC. Although Ramshot powders are IME much more forgiving than Hodgdon equivalents in this respect, this type of propellant (ie 'ball') tend to operate better at full working pressures. It's quite common when working loads up to see a considerable hike in MVs and reduction in velocity spreads as the charge weight approaches maximum and combustion becomes more efficient.

Still, if you're happy with the results, this is all by the by as effect on the target (or whatever else is being shot at) is the name of the game.

Thanks for the explanation Laurie. Mossberg do chamber their rifles for 5.56, so your freebore explanation could be correct.

If I really do want to get up to the hgher velocities it looks like I need to experiment with seating depth, but as i'm happy with the accuracy with these loads, I'll leave well alone for the time being.

Daryll.