A buddy of mine has two 338-378's built on P-14'S and swear's by them.BUT I have also seen a picture of one that came apart.And it was not pretty.My friend has had one of his for quite a few year's now with no probablem's.
I have three P-14's built up into rifles. One is a .300 Win. Mag., one is a .338-.378, and one is a .510-.505 Gibbs. The actions seem to be sufficiently strong and stiff. The .300 Win. Mag has a 1.25" dia. untapered barrel that is free floated from about an inch in front of the recoil lug, and it is a sub-1/2 MOA rifle that threatens to shoot 1/4 MOA (couple groups that small). I can't get better than 1.25 MOA out of the hunting weight barrel (free-floated from an inch forward of the recoil lug) on the .338-.378. I guess I've just got to try a lot harder. The .510-.505 Gibbs is early in development, and shooting 700gr. M2 ball through a turned-down 32" long 1.375" dia. M2 .50 BMG (also free-floated from the recoil lug forward) barrel, it looks like it may shoot 3/4 MOA. All of these rifles are single shots because either the ammuniton is too long or I'm too lazy to open up the rails. I would caution you to watch the throat length on your reamers, or the ammunition won't have bullets close to the rifling and fit in the magazine, and your bullet choices will be limited to the heavier, longer bullets. Anybody know where to get P14 actions cheap?
The P14 action is a very strong action. Bill Miller (Gunsmith) purposely blew one up as a test using 4831 powder and a 250 gr bullet in a 378 case.
It held up remarkably well. We found that the Remington and Winchester P14 actions were a tad stronger then the English made P14.
Walt Bryan (Williamsport Shooter) uses them on some of his match guns as do others.
As in any action, steel fatigue sets in from years of use and could be a problem.
I use to buy these 10 at a time and take the barrels off and sell the actions. I have one gun now with a P14 Remington action on a magnum case. From time to time you see a P14 action for sale.
I would caution against using the P14 or P17 for any case with a head larger than the regular Mag. The larger the head, the greater the pressure on the bolt face and hence the lugs. The Ps 14 and 17 have less lug sear surface than the Rem. 700. Cautions related to using the .378 size case in the 700 would apply more so to the Ps.