bullet depth.

[colonyfirearms]

I am loading 117 gr 25 WSSM.
I want to shoot this in as AR-15.
To fit the magazine the bullet needs to be set with the Cannelure groove in the neck of the case about .100/inch below the mouth of the case.
The bullet is still full size diameter at the mouth of the brass.
Will this cause any problems.

[Catfish]

Seating the bullets extra deep lowers the capacity of the case and your loads will get hot with less powder. On the first 90 degree day start low and test fire as you work up capcity. If your bullets are a long way off the lands it will help with the presure, but will not help accuracy. The only way to know is to take it to the range and try it.

[Rapier]

Old rule of thumb about accuracy and consistant pressures. You should never seat the base of the bullet below the base of the beck with a flat base bullet. You should never seat the base of a boat tail bullet so that the top of the boat tail is below the base of the neck.

The trick, which is not really a trick: in order to gain bullet weight in a magazine fed gun where the interior of the magazine is the limiting factor, is to go to a boat tail and set the bullet down until you get to the top of the boat tail and still have .050 less than the interior deminsions of the magazine. Sometimes you can fudge the .050 a little.

Measure the magazine length inside. You measure the case length, the neck length and the bullet length, less the BT length. Add the case length - neck to bullet length-BT and see if you can get a .050 difference off the magazine length, if not, go to a different design or lighter bullet.

I use this method in designing wildcat cartridges to find a cat design that should work well. It is exactly how I worked the 120 Sierra MK into my AR-10T chambered for the 260. Once I got the OAL to fit the mags I throated the chamber to .003 off the lands using that setup.
Ed