Yes, the M69 cyl is long enough to handle the Keith bullet in Magnum brass crimped in the crimp grove -- it is real close to the face of the cyl, so make sure you have good neck tension and a strong crimp to prevent bullet movement.
For anyone that might be interested, here are some measurements comparing the M69 to a M629 Mtn Gun:
629 Mtn Gun weighs 39 oz
M69 L Frame weighs 37 oz
629 Mtn Gun Cylinder Diameter: 1.70"
M69 L Frame Cylinder Diameter: 1.56"
629 Mtn Gun Cylinder Length: 1.705"
M69 L Frame Cylinder Length: 1.670"
(Note: The M69 cylinder is 1.670 inches and isn't recessed for cartridge rims -- a 1.712" hand load fits, but is right at the face of the cylinder. Lymans No 49 edition shows OAL w their 425421 Keith Bullet is 1.710". A .429 minus plug gauge will enter all throats a .430 plug will not.)
629 Mtn Gun Frame Window - Height: 1.735"
M69 L Frame - Frame Window Height: 1.600"
629 Mtn Gun Frame Window – Width: 1.880"
M69 L Frame - Frame Window Width: 1.820"
629 Mtn Gun Cylinder Wall at Narrowest Point: .095" -- (.0900" between chambers)
M69 L Frame Cylinder Wall at Narrowest Point: .060 " -- ( .130 " between chambers)
(629 Cyl Bolt Cuts are over the chamber while M69 bolt cuts are between chambers)
629 Mtn Gun Frame Top Strap – Width: .665"
M69 L Frame - Frame Top Strap Width: .665"
629 Mtn Gun Frame Top Strap – Height: .220" to top of frame (includes sight mortiss)
M69 L Frame - Frame Top Strap Height: .210" to top of frame (includes sight mortiss)
629 Mtn Gun Barrel Shank OD: .630"
M69 L Frame - Barrel Shank OD: .620"
629 Mtn Gun Frame Over Barrel OD: .900"
M69 L Frame - Frame Over Barrel OD: .880"
629 Mtn Gun Trigger Reach: 3.10" (same grips for both 629 & M69)
M69 L Frame - Trigger Reach: 3.00 "
Brian Pearce -- Bolt notches weakest point - M69 thicker than M629 at this location
Paul
