.410 Overbore Project


I would like to incorporate your thoughts into one last .410 project..I believe you postulated that an “overbore” and longer forcing cones could be beneficial here. SAMMI spec is .420 I believe. Forcing cone, whatever……..Our brain trust has theoretically determined that .450 is possible ( wall thickness not being an issue) with modern shells, forcing cone 5″ nominal ( we have access to an aerospace grade machine shop) this facility finishes to 1/2 lightwave surface finish if required. Barrel length discussed 30″ nominal up to 32″ maximum. I have been doing by best to wear out a pair of pre war Francotte .410s for the last 45 years or so, after many thousands of rounds it may be time to think of retiring them, therefore, this project. If you had your “druthers” what are your thoughts and comments .



Dear Peter,

This is new territory for me. Perhaps Briley or Kolar has tried something similar for someone and might have some information.

My main concerns would be whether today’s .410 plastic wad will be able to obturate 1) in such a long forcing cone, and 2) in a .450″ bore. You mention that the SAAMI .410 bore specification might be .420″. I usually see .410″ as nominal for the 410, but you may well be right if SAAMI allows a range of overage. The .410 bores I’ve seen measured are often under .410″, though manufacturers will differ here. My point is that a .040″ overbore is a great deal in a 12 gauge gun and a humongous amount in the .410. That would make me doubly concerned about wad skirt obturation.

But it certainly is an interesting project. The little 410 needs all the help it can get. Make sure that you do some accurate pattern testing before and after and let us know what you find. This could be a lot of fun. I’ve got a few Model 42s that just sit there and smirk at me. I’d love to teach them a lesson.

Best regards,
Bruce Buck
Shotgun Report’s Technoid

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