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  6MM Rat & FAT RAT Information Guide     History of the cartridge-     I had seen other 6mm Grendel Improved cartridges out there but there was not a source to just buy a barrel and parts to build your own. So I decided to design my own reamer and have a set of custom dies made for it. I had done this before for an AR-10 project that I did in a .243 wildcat cartridge. Lonnie Hummel of Horndays custom dies is very knowledgeable and helpful on this kind of project.   I worked out the reamer print for the 6mm RAT with the help of Lonnie Hummel from Horndays custom dies, John Hollinger from White Oak Armament and Dave Kiff from PTG. The reamer was designed to be used with Alexander Arms 6.5 Grendel Brass and for a gas gun. The dies were also designed for a gas gun. After the reamer and dies were made, John Hollinger sent me a test barrel to do the initial testing and everything looked good. John Hollinger asked me if it would be alright if he chambered other customer barrels in this cartridge and suggested I have a batch of dies made up for the cartridge. I designed this cartridge initially for myself, but others have wanted to use it. I am glad to help other shooters and pass on the information that I have learned.   Greg Krebaum       Specifications   -Max case length 1.525 -Trim brass at 1.520 (Probably will not ever have to trim the brass because of the 40 degree shoulder.) -Head Space 1.266 -Water capacity 36.7- 37.0       Fireform loads are 30.0 Grains of Re-15 or 28G of H4895 with 105-108 bullets   Too light and you risk not completely forming the brass or even cracking the neck  

Load Data

     

Specifications 

-Max case length 1.525  -Trim brass at 1.520 (Probably will not ever have to trim the brass because of the 40  degree shoulder.) -Head Space 1.266  -Water capacity 36.7- 37.0   -OAL to the lands   108 Berger 2.338   105 Lapua 2.335   105 Berger vld 2.389    -Magazine length for the c-products mags are 2.28 – 2.29 oal   You can cut the front out of your mags if you want to get a longer oal for your   magazine load    

Reloading 

- Chamber was set up so 108 Bergers, 105 Lapua, 107 Sierra, boat tail juncture would be at or slightly below the neck shoulder juncture of the brass. 105 VLD will be approximately .040 above the neck  shoulder juncture.       

These loads will not work with the RAT 

Always remember to start low and work up to the  MAX loads  

Further load testing will be done as time goes 

The case must be resized from 1.264” min to 1.268” max using a stoney point “B”.350 head space bushing          Fireform loads are 30.0 Grains of Re-15 or 28G of H4895 with 105108 bullets   Too light and you risk not completely forming the brass or even    cracking the neck             6 MM FAT RAT BULLET OAL LENGTHS   

in a new chamber   

  BC OAL to OGIV OAL to TIP Max Mag Length Jump 105   Berger Hunting VLDS 532 1.882 2.37 2.260 0.089 105   Berger Match VLD 493 1.889 2.370 0.080  108   Berger Match VLD 511 1.889 2.337 0.047  95   Sierra HPBT 480 1.922 2.380 0.090 107   Sierra HPBT  527 1.926 2.372 0.082  85   sierra HPBT GameKing 311 1.885 2.175 0.000   

For the V-FAT RAT the OAL length is .114 shorter than the standard Fat Rat 

( start with one grain less than listed and watch for pressure signs as you gradually increase towards the max loads listed)    Lee's FAT RAT Long Range Loads   

  

Start with 1/2 grain less than these MAX loads and work up to these, watching for pressure signs. Keep in mind that all barrels are not alike 

6MM FAT RAT MAX Long range Loads  • 107 Sierra HPBT with 30.0 RE-15 ------------------2820 fps  • 108 Berger Match with 30.0 RE-15----------------- 2845 fps  • 108 Berger Match with 30.5 Varget-----------------2846 fps  •   

These loads below showed no pressure signs and could be increased by 2 tenth increments until pressure signs show up 

• 95 Sierra Match with 28.1 H4895------------------- 2792 FPS Min Load  • 95 Sierra Match with 28.7 H 4895------------------2860 FPS  • 105 VLD Berger with 28.2 Grains H4895  • 90 Berger 28.7 Grains H4895