Any 356GNR GP100 owners?

I bet it will be a very short service life due to the amount of pressure that is forcing out between the face of the forcing cone and the face of the barrel.
A GP100 conversion would be the last thing I would have converted to that caliber.
It's made from a .41 magnum case necked down to a .356 caliber bullet and is shooting that bullet at almost 30-30 velocities.

First off I wouldn't worry about the barrel as much as I would worry about the cylinder letting go unless the cylinder is replaced and made out of something like Carpenter's Custom 465 Stainless.
Even if the cylinder was replaced with one made from some metal that would handle that kind of pressure, that has chambers that thin, that would fit in the GP100 frame, the face of the forcing cone of the barrel would get burn off in no time at all.

Now factor in the weight of a GP100 shooting a round like that. Factor in frame stretching.

All of the guns that came up in a quick Google search of the caliber are single shot pistols like the Contenders that don't have a barrel to cylinder gap.

I have no idea about the barrel steel Ruger uses in the GP100, That is for the custom gun shop to be responsible guaranteeing, if you trust them.

As far as someone chambering a GP100 that was designed for .357mag pressures at .357 mag speeds, for a round that would shoot a necked down 41mag case in a revolver with a cylinder of that diameter, and think it will have a long service life, they must be smoking Crack.

Now I have my fire proof suit on so Flame away!

Ok, a couple of misconceptions here.

A 356 GNR will exceed 357 mag ballistics at lower pressures than a 357 mag. A six shot 356 GNR has wider chambers walls between cylinders than a 7 shot GP100 has. Gary Reeder is a very competent gunsmith and his conversions are all high quality and well tested way in excess of the loads he specifies.

You can rest assured you will get a very safe and well functioning firearm when you get a GR conversion, significantly improved over the standard unit.

I’m not affiliated with GR in any way, but I do have a couple of his firearms and they are top notch.

98s1lightning said:

As for the limited 10mm GP' s rugers making its a cool idea but I'll pass on moonclips and rimless brass in a Revolver. But the heavy 10mm hot stuff is proof the GP can take a little more abuse than a hot n heavy 357 load. The 10mm can produce roughly 100-200 more foot lbs of energy I reckon.

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A 10mm has a smaller case capacity than a 357 mag, and at max loads cannot exceed .357 mag loads for the same bullet weight.

A 10mm GP can be rechambered to 10mm mag which does give it an edge over. 357mag.

98s1lightning said:

Tightgroup I see guys on forums that own these gp conversions and Blackhawk conversions on these medium frame models.

It sure makes for an interesting discussion.

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I'll bet it does.

tightgroup tiger said:

It's made from a .41 magnum case necked down to a .356 caliber bullet and is shooting that bullet at almost 30-30 velocities.

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It’s not making that speed in a GP100, not by a long shot. Gary’s data for an 8” Blackhawk is 1815-1850 max for a 158grn pill, a good 500-600fps slower than what can be attained with a Dirty Thirty. Even given a long barrel, there’s simply not enough case capacity there to reach the same performance - the 356 case is giving up about 15grn H2O to the .30-30, even with higher pressure, it’s a gap too wide to close.

I push a 180 to 1900-2000, depending how frisky I want to get, with a Redhawk in .357/44 Bain & Davis, with about 4grn H2O greater case capacity - and a hell of a lot more cylinder wall thickness than a GP100. I cheated my way into a 356 GNR in a Contender, and wouldn’t quite keep up with my 7.5” B&D.

Personally, the 356 GNR is right where I’d want to be in a GP100, and equally, the GP100 right where it should be. All the power it can handle, and not a drop more. Also, personally, I’d shoot it with glee until it rattled loose, and then have it tightened up and start all over.

I’ll confirm, there is a gross misconception above that these operate at substantially higher pressures than .357mag. Quickload does the math for us - we can put a heck of a lot more powder under these bullets without as much pressure. Have to remember - revolver cartridges will reach their pressure peak typically while the bullet base is still within the cylinder, and the larger cases have larger internal volumes - we can put proportionately that much more powder into the case which does more work for us down-bore, without raising the maximum pressure peak.

The big case, small bore, bottleneck revolver cartridges do bark a lot of fire and hate - but do remain to be rather mild on the business end.

Firstly, this idea that "double the energy" has any meaning is just false. It 'may' increase the effect of 'some' bullets while pushing others beyond their limits, thereby reducing effectiveness. Energy is only useful as a marketing tool to sell velocity. For measuring the terminal effect of a given cartridge, it is useless.

With that said, I'll take a heavy bullet at moderate velocity out of the .44Spl over anything you can stuff in the .357. Same for the 10mm. Now that we have the 190gr Lehigh coming to market, it is a definite step up from the .357. It's not ideal but rimless cartridges and moon clips aren't much trouble with the right tools.

I could be wrong but it seems to me that a .356 in a six shot GP is cutting into the safety margin. I'm sure it would be fine as suggested above to shoot it loose and start over again but the idea of the .41Mag in a six shot GP makes me a bit nervous. A five shot, sure enough.

Lastly, I wouldn't let Gary Reeder iron my underwear, much less work on one of my guns. If you're gonna spend a bunch of money to make the GP more effective, send it to a proper gunsmith to have it built into a five-shot .41Mag.

I hate to be that guy, but I wouldn't consider it on a gp100. Thats the forcing cone of the 1 gp100 I owned. Bought it new and in the 5k rounds I put through it (all factory 158 gr loads) it went back to ruger 3 seperate times.

Such a beast begs for a half lug 8” barrel.