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Sample,
I have to strongly disagree with that Hornady rep! Three grains of powder to compensate for 5" less barrel is insane!
Most of the information I've read states that loads are pressure limited, and that barrel length will only effect the velocity obtained with those loads, at the intended safe pressure.
Adding powder has never been recommended to compensate for short barrels as he suggested, otherwise all load manuals would be categorized by barrel length, as well as bullet weight!
It's common knowledge that powder can be increased with a lighter bullet, because a given charge will develop less pressure with the lower resistance of a lighter bullet.
The fact is, that gunpowder reaches maximum pressure almost instantly after ignition, and begins to decrease almost as soon as the bullet begins to move down the barrel. The reason pressure decreases, is that the containment space is increasing as the bullet moves, and most of the powder is already burned by that time! Velocity increases if the bullet is accelerated for a longer time (i.e. longer barrel), but you can also reach a point where containment space is too large (i.e. too long a barrel), and the acceleration ceases, or even decreases (velocity loss).
As to pressure signs, read some of the early tests run by John Linebaugh and Ross Seyfried with high velocity loads, using pressure signs to watch pressure. Seyfried stated recently that he exceeded 300 Win Magnum pressures in his Blackhawk handgun, and saw no signs of excess pressure! That's cylinder burst pressure in most cases!
Slow powder does not increase safe pressures, but builds up to that pressure slower, so the bullet can accelerate longer.
I see you have read the same information on the Linebaugh site. Notice that John recommends an absolute maximum load of 20.5 grains of 2400, and that is behind a 260 cast lead bullet that develops far less pressure than your Hornady XTP bullets of the same weight!
Not trying to scare you, but it would be wise to at least check the velocity your are getting, as velocity is a pretty reliable indicator of pressure. If your velocity is comparable to published maximum loads for your bullet/powder combination, your probably cooking at safe pressure.
Take care,
Bill
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