The UK's Finest Independent Forum for Shooters and Gun Owners. Est 2010.

12 gauge has a SAAMI max chamber pressure of 11500psi. .410 has a maximum SAAMI pressure of 13500psi.

I can't dispute the figures, but the gunsmith who declared the gun out of proof wasn't fazed by my decision to obtain some .410 inserts.

Breech pressure peaks for a fraction of a second and and then begins to fall as the charge moves, this takes place inside the inserts.
As soon as the charge leaves the .410 tube, the pressure will drop significantly due to the greater volume of the 12 gauge bore.

That's the theory anyway!

If a .410 propellant is allowed to fill a 12bore space the pressure will be a lot less I should think. Without going back to the books I think it's the inverse square rule.

Mattnall wrote:If a .410 propellant is allowed to fill a 12bore space the pressure will be a lot less I should think. Without going back to the books I think it's the inverse square rule.

That might be true, however the pressure will be at maximum whilst the gas and projectiles are still within the inserts. Then as they leave the end of the insert and into the original bore, the increase in volume will be very small (although increasing rapidly), so parts of the original bore could be subjected to high pressure.

The .410 adapter should be designed to accept the maximum pressure of the cartridge should it not? Consider the wall thickness of the adaptor/the diameter difference between 12g & .410

Ergo, the only pressure acting upon the parent gun would be in recoil I would have thought…

Tommygunn wrote:The .410 adapter should be designed to accept the maximum pressure of the cartridge should it not? Consider the wall thickness of the adaptor/the diameter difference between 12g & .410

Ergo, the only pressure acting upon the parent gun would be in recoil I would have thought…

Tha adapter will accept the pressure but some are just 75-100mm long meaning that at te end of the adapter it is open to the normal bore and that will be subjected to the same high pressure, or maybe a little less but still high. And older 12"/300mm long one may well keep pressures down. Would I chance it - hard to say but I may well say NO.


https://www.firequest.com/HV410.html - all they do is effectively convert the .410 diameter to 12g.


CAUTION is needed and a real gunsmith needs to look at teh barrels and adapters being considered.

The bore of a shotgun is smaller that the chamber. It is the forcing cone that allows the transition. Average chamber pressure at the point of ignition becomes pressure in the barrel, albeit decreasing as the projectile moves along the bore, and it decreases fairly rapidly. And in the big scheme of things, the neck of a .410 cartridge is .455” and a 12 gauge bore is a nominal .729” not a huge amount of free space. Certainly not akin to entering Twickenham from the dressing room!

Gas port pressure, the point in the bore where gas would be bled off to operate a semi, can be 40% or so of chamber pressure. So we could be talking half way down the bore, even with increased internal diameter, coils still be around 5000psi.

This inherently weak, 100 year old, Damascus barrelled gun, is already severely pitted. Even a phenomenally lower pressure relatively, is still capable of ripping apart a structurally compromised barrel. Nah, it would still turn into a safe queen for me. Kept only for sentimental reasons with no prospect of it ever shooting again.

I’ve done more or less the same thing with an 1897 vintage Marlin 1893 in .32 Winchester Special. Ultimately rode hard, put away wet…

Polchraine wrote:

Mattnall wrote:If a .410 propellant is allowed to fill a 12bore space the pressure will be a lot less I should think. Without going back to the books I think it's the inverse square rule.

That might be true, however the pressure will be at maximum whilst the gas and projectiles are still within the inserts. Then as they leave the end of the insert and into the original bore, the increase in volume will be very small (although increasing rapidly), so parts of the original bore could be subjected to high pressure.

The adaptor is built and proofed to take the pressure of the cartridge not the 12ga barrel.

Once in the 12ga bore the increase in area/volume is far from very small, it is actually quite significant.

The area of a 12ga section of barrel is over 3 times the area of a .410 barrel and the volume will be similarly (at least 3x) larger, simply speaking Boyle says the pressure will be over 3x less. Using the max P of 13500psi for 410 (given above) the max pressure in the 12ga barrel would be nearer 5000psi, well under the 11500psi mentioned above.

Mattnall wrote:

Polchraine wrote:

Mattnall wrote:If a .410 propellant is allowed to fill a 12bore space the pressure will be a lot less I should think. Without going back to the books I think it's the inverse square rule.

That might be true, however the pressure will be at maximum whilst the gas and projectiles are still within the inserts. Then as they leave the end of the insert and into the original bore, the increase in volume will be very small (although increasing rapidly), so parts of the original bore could be subjected to high pressure.

The adaptor is built and proofed to take the pressure of the cartridge not the 12ga barrel.

Once in the 12ga bore the increase in area/volume is far from very small, it is actually quite significant.

The area of a 12ga section of barrel is over 3 times the area of a .410 barrel and the volume will be similarly (at least 3x) larger, simply speaking Boyle says the pressure will be over 3x less. Using the max P of 13500psi for 410 (given above) the max pressure in the 12ga barrel would be nearer 5000psi, well under the 11500psi mentioned above.

No, it will not.

At the point just after the wad leaves the end of teh adapter and is maybe 1mm away, from the end of a 100mm long adapter the incremental volume will still be vey small. Yes, once it is 300mm down the barrel the pressure will be well down, but instantaneously at the break out point it will still be high.

Mattnall wrote:
… simply speaking Boyle says the pressure will be over 3x less. Using the max P of 13500psi for 410 (given above) the max pressure in the 12ga barrel would be nearer 5000psi...

So the same figure I came to, there or thereabouts. Do you reckon that 5000psi in a compromised Damascus barrel would be safe? I’d still be erring on retiring it. Just not worth the risk.

Polchraine wrote:

Mattnall wrote:

Polchraine wrote:
That might be true, however the pressure will be at maximum whilst the gas and projectiles are still within the inserts. Then as they leave the end of the insert and into the original bore, the increase in volume will be very small (although increasing rapidly), so parts of the original bore could be subjected to high pressure.

The adaptor is built and proofed to take the pressure of the cartridge not the 12ga barrel.

Once in the 12ga bore the increase in area/volume is far from very small, it is actually quite significant.

The area of a 12ga section of barrel is over 3 times the area of a .410 barrel and the volume will be similarly (at least 3x) larger, simply speaking Boyle says the pressure will be over 3x less. Using the max P of 13500psi for 410 (given above) the max pressure in the 12ga barrel would be nearer 5000psi, well under the 11500psi mentioned above.

No, it will not.

At the point just after the wad leaves the end of teh adapter and is maybe 1mm away, from the end of a 100mm long adapter the incremental volume will still be vey small. Yes, once it is 300mm down the barrel the pressure will be well down, but instantaneously at the break out point it will still be high.

OK.