6mm Fat Rat

[cruddymutt]

Is anyone shooting/reloading this caliber? I have a barrel/bolt/die set on the way and Im wondering if anyone is using 7.62x39 brass to form Fat Rat.

[Wrangler John]

From the case drawings it appears to be theoretically possible. The rim diameters are close, .449" for the 6.5 Grendel parent case, and .445" for the 7.62 x 39 case, with the case head diameter .443" for both. The shoulder to neck junction is a bit shorter on the 7.62 x 39 than the 6.5 Grendel, which shouldn't much of a problem in that headspace will be set with a double shoulder from the neck draw operation. Case length for both are close with the 7.62 x 39 being .009" longer. Because the case will fireform to a more "improved" shape it will shorten slightly anyway, that extra neck length is insignificant, or it can be trimmed if your die set has a form/trim die.

I use 7.62 x 39 brass to form into 6mm PPC without any problems, and it forms nicely. I do have an old lot of Remington 7.62 x 39 brass that was manufactured with a small rifle primer pocket specifically for forming to 6mm PPC. However, I have been tinkering with wildcats for over 40 years, which makes me completely mad and quite fearless. I still covet a .243 WSSM upper, as it seems so radical as to be entertaining.

One thing to contemplate would be to have Hornady's Custom Shop make up a hydraulic forming die once you have fired cases for measurement. That way you'd mechanically form the cases and then hydraulically form them to conform to the chamber prior to firing. It's messy using water as the fluid, but I get nicely formed Ackley Improved cases without having to fireform. It's all part of being a rifle looney.

You might want to give a call to this outfit, if it was where you ordered your parts from, just for confirmation: http://www.accurateandreliablerifle.com/products.html

Good luck, it sounds like a fun project.

[Cypriss32]

http://forum.snipershide.com/snipers-hide-semi-auto-rifles/199550-new-build-6-5-grendel-wildcat-fans-6mm-fat-rat.html

[cruddymutt]

http://forum.snipershide.com/snipers...m-fat-rat.html

That's the thread that I read that eventually led to me ordering the barrel. I posted the same question there but it never hurts to hear other opinions I figure.

[cruddymutt]

From the case drawings it appears to be theoretically possible. The rim diameters are close, .449" for the 6.5 Grendel parent case, and .445" for the 7.62 x 39 case, with the case head diameter .443" for both. The shoulder to neck junction is a bit shorter on the 7.62 x 39 than the 6.5 Grendel, which shouldn't much of a problem in that headspace will be set with a double shoulder from the neck draw operation. Case length for both are close with the 7.62 x 39 being .009" longer. Because the case will fireform to a more "improved" shape it will shorten slightly anyway, that extra neck length is insignificant, or it can be trimmed if your die set has a form/trim die.

I use 7.62 x 39 brass to form into 6mm PPC without any problems, and it forms nicely. I do have an old lot of Remington 7.62 x 39 brass that was manufactured with a small rifle primer pocket specifically for forming to 6mm PPC. However, I have been tinkering with wildcats for over 40 years, which makes me completely mad and quite fearless. I still covet a .243 WSSM upper, as it seems so radical as to be entertaining.

One thing to contemplate would be to have Hornady's Custom Shop make up a hydraulic forming die once you have fired cases for measurement. That way you'd mechanically form the cases and then hydraulically form them to conform to the chamber prior to firing. It's messy using water as the fluid, but I get nicely formed Ackley Improved cases without having to fireform. It's all part of being a rifle looney.

You might want to give a call to this outfit, if it was where you ordered your parts from, just for confirmation: http://www.accurateandreliablerifle.com/products.html

Good luck, it sounds like a fun project.

Yes I ordered the barrel from Lee at Accurate and Reliable. I asked him about using 7.62x39 brass and he told me he had never tried and to let him know how it worked LOL. I figured since 7.62x39 can be formed into 6.5 Grendel and 6.5 Grendel is the parent for 6mm Fat Rat it would work. I know that Grendel brass uses a SRP and has an undersized flash hole which is claimed to be the "secret" to its accuracy. Im leaning towards using the 7.62x39 brass as practice loads so my son can get some seat time behind the rifle since Im building it for him. Ill save what little 6.5 Grendel brass I have to use for hunting loads until I find some more.