6MM Rat & FAT RAT Information Guide



6MM Fat Rat reloading guide

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


-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


-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 105-108 bullets

  Too light and you risk not completely forming the brass or even cracking the neck   

  in a new chamber

                                               BC    OAL to OGIV          OAL to TIP       Max Mag         Length Jump

105 Berger Hunting VLDS          532     1.882                   2.37                   2.290                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


Always 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


6MM Rat case on left with 107g Sierra
       6.5 Grendel case on right
This page was last updated: October 26, 2015
Developed by Greg Krebaum in Kansas, the 6 MM Rat is based on the 6.5 Grendel case necked down to 6MM with the neck blown forward aprox .075 at a 40 degree angle.
 This rifle is built on an AR-15 flat top platform with quality American manufactured parts, the 6MM bullets have a very good B.C. The 6 MM and 6.5 caliber bullets have been dominating competition shooting the last few years. Why not get with the program and start shooting the equipment that will perform the best

 This upper is built to utilize a service rifle lower, for those that would like to shoot both Service Rifle and Match Rifle competitions and not have to own 2 separate lowers with 2 different trigger weights.
 It has tested to be sub 1/2 minute of angle with the 107 G Sierra  bullets.  

Match Rifle Hand Stop    $50.95
Just tell us your tube dia. & we make it to fit
with sling swivel  
#AR MR293
Match rifle AR-15 Flat top upper only (Get Quote)
Choose caliber (6MM, 264LBC or 223), barrel make, twist, and color
A4 stripped upper  W/M4 ramps $102.00

#AR U122

IN stock
A2 Stripped Upper  $96.00
#AR U124
Lower Receiver Parts kit "less trigger"  $40.00 
#AR L63

with trigger only 59.95 # AR L64 

Prices subject to change
Click here for more 
   FAT  RAT Info on Loads and Velocities
email me
email me
My work has a 100% guarantee
These barrels will shoot sub 5/8 minute of angle- guaranteed- most will be less than 1/2 min
Bolt Carrier  $66.00
                       #AR B207
223 complete bolt  $50.00                                      #AR B208
             AR-15 PARTS
Firing pin                              AR B202        $6.80
Cam pin                               AR B201         $5.00 
Firing pin retaining pin            AR B203        $0.75
Forward assist kit                  AR B211        $12.00
Ejection port cover kit             AR B212        $8.00
Extractor                               AR B213        $15.00  
Flash suppressor                   AR U214        $8.00
Barrel extension  W/M4 Ram  AR B206        $20.00 
Charging Handle complete      AR SR200      $19.95
Stainless Steel .223 Wilson 1-7 twist preban barrel with fitted bolt, barrel ext,
Wylde comp chamber & 11 deg. crown ONLY $ 266.00  PT # AR 404 (barrel only) $216.00
 223 complete bolt carrier assy  $119.00
                          #AR B209
Pin your rear sight with purchase and installation of a new barrel for an additional $49.95

Warranty - We guarantee our products to be free of defects in material and worksmanship. We guarantee the premium barrels we use will shoot 5/8 min of angle or better with quality factory match grade ammunition comparable to Federal Match Ammunition, or we will replace the item or refund the purchase price at our discretion.

Any type of Finish Lapping, Fire Lapping, Black Star or any process of trying to polish the bore after it had left our shop, voids any/all warranties by Accurate and Reliable Rifle

Firing hand loaded ammunition voids our Warranty 

We reserve the right to see our products for inspection and/or the firearm including testing, before replacement/refund of any upper or barrel

email me
AR Parts
Tell a friend about this page
Prices subject to change
Post ban SS SR .223 Wilson 1-7 twist with barrel ext w/wind adj front sight & fitted bolt, Wylde comp chamber & 11 deg. crown ONLY $296.00.  #AR 406 (barrel only $205.00)
See "Build your Upper page"
With purchase of Accurate and Reliable Rifle match grade,Krieger or Wilson barrel,
I will rebarrel your upper for $55.00 labor,
this includes 11 degree target crown, Wylde chamber machined to close tolerances and your sight base modified with wind adjusting set screws ( -$10.00 if your sight base is already modified)
6MM FAT Rat     $582.00 
Krieger premium SS 1-8 twist with Barrel Ext, fitted 6.5 Grendel bolt, 11 degree target crown AR-15 profile w/out gas block and gas tube your choice of barrel length up to 28"    PT # AR 421    Barrel & Ext only $500.00
Delux Bi-Pod adjustable stop, accepts 1/4-20 QD Stud    $35.70
sits 3/4" below Free Float Tube will fit Freeland and Anschutz style accessory rails
Just tell us your tube dia. & we make it to fit

#AR V33
 Bi-Pod Stud will fit Freeland and Anschutz style accessory rails. $9.95

#AR V34

email me
A&R Ultra Match Rifle chambered in 264 LBC utilizing an Alexander Arms Billet upper with side charging handle. This Upper is meticulously assembled with top quality parts to produce an upper receiver that is capable of extremely tight groups. Guaranteed sub 1/2 min, The White Oak Arms Match Rifle Float Tube will accept Anschutz accessories as well as the A&R Rifle match rifle hand stop.
 The Alexander Arms billet upper is a beefy, well designed squared upper that resists flex from long barrels, includes chromed carrier and side charging handle.
mouse over pics for enlarged view
264 LBC
Only $1249.00
Lead order time Approx 8 weeks
Buttstock Weight  $24.00
#AR 501
Fore-end weight    $29.00
#AR 502
Post ban Premium SS .223 Krieger 1-7.7 twist with fitted bolt, barrel ext, wind adj front sight base, Wylde comp chamber & 11 deg. crown ONLY $570.00  PT # AR 411
6MM FAT Rat            $385.00  
Wilson SS 1-8 twist with Barrel Ext, fitted 6.5 Grendel bolt, 11 degree target crown AR-15 profile w/out gas block and gas tube your choice of barrel length up to 26"        PT # AR 438 Barrel & Ext only $305.00
Pre ban Premium SS SR .223 Krieger 1-7.7 twist with fitted bolt, barrel ext, wind adj front sight base, Wylde comp chamber & 11 deg. crown ONLY $585.00 PT # AR 413   barrel & fitted bolt only $535.00 (Barrel only $485.00)
6.5 Grendel Bolts Alexander Arms
AR B109                   $81.00
Modified Flat Top AR-15 to accept National Match Carry Handle with True 1/4 or 1/2 min National Match sights (STRIPPED RECEIVER ONLY)
AR SR612  $138.00
National Match Carry Handle with 36 min of elev adjustment, pinned sight for true elev tracking 1/4 or 1/2 min & 3 hooded apertures
AR SR613  $230.00
(UNPINNED only $180.00) 
Upper must be machined to accept the National Match sight as shown at left
White Oak Varmint rifle float tube 
AR V130       $125.00

Tube is not slotted, but instead has a bi-pod stud which can be used to attach a bipod. (black only)
20 round Brownell AR 15 magazines. I have used these myself. Now there is no nee to buy used Mil surplus mags and take your chance on if there OK or not. These work great 
Part # AR 500  $14.95
Extra-wide steel swivel accepts slings up to 1 5⁄8" wide   AR MR31  $9.99
2 1/4"  Black out of STOCK
Modified hand guard set to fit CMP legal Free float tube  AR U340  $25.00
These plugs are designed to block gasses coming to the rear out of the charging handle recess. There is no longer any need to silicone the charging handle 
replaces the rear charging handle for side handle uppers
IN STOCK   " Gas Blocker Plug"
AR U199  $4.99
AR S245       $139.95

The Original SR MicroSight, an aiming aid consisting of an integrated hood, lens, and aperture for the AR-15 service rifle. 

What it does: Since the eye is only capable of focusing on one plane at a time, and the front sight and target are in separate planes, conventional wisdom says they both cannot be in focus at the same time. Fortunately, Stallings Machine has innovatively extended recent discoveries in diffraction optics to create an aiming aid called the Original SR MicroSight providing multifocal capability. By careful design and fabrication, a single miniature optical element provides simultaneous focus on both the front sight and target. 

How it works: The MicroSight is a special application of a more general device called a Phased Zone Plate. The MicroSight attempts to confine the light into two focal lengths corresponding to the target and front sight. 

Installation: The Original SR MicroSight hood replaces a standard AR-15 hood (1/4-32 threads) as found on most NM AR-15 rear sights. If you have questions about whether or not the MicroSight will fit your AR-15, contact Stallings Machine. 

Application: Users must have good natural or corrected distance vision. It is very important that the user's vision be correct for distance viewing since the MicroSight provides the target image by passing a portion of the light to the eye unaltered. This is of course contrary to the conventional approach of correcting vision to provide a clear view of the front sight. The MicroSight physical parameters are established by the sight radius. A MicroSight designed for a short barreled weapon like the AR-15 will not work well on a Palma Rifle and vice versa. 

One size fits all: Once your vision is corrected for best distance viewing, you do not need to select from different MicroSight configurations. A single combination of hood, lens, and aperture is offered to satisfy the needs of all shooters. The aperture opening in the MicroSight is approximately .040. While many shooters use a smaller rear aperture because the increased depth of field helps them bring the front post into better focus, the MicroSight achieves sharp front sight focus without depending on the increased depth of field (and the resulting loss of transmitted light) of a smaller aperture. If you REALLY feel you need something other than a .040 aperture, contact Stallings Machine. 

Legality: The hood that houses the MicroSight is well within the size allowed by the rules as they are written. 

 Match Rifle Premium SS .223 Krieger 1-7.7 twist with fitted bolt, barrel ext, Wylde comp chamber & 11 deg. crown 28" long ONLY $550.00  PT # AR 419
Introducing the MicroSight
White Oak Match Rifle Float tubes AR U131 only $145.00
These are 14.5" long with your choice of 2" or 21/4" Dia
Match rifle float tubes are slotted for Anshutz-style handstops, slots are 10.5" long with wide opening at either front or back, "Customer preferance"  Please specify front or back wide opening, color and diameter when ordering! 

6MM Rat OAL guage to fit most OAL guages
AR R288 only $9.95  
out of STOCK
Prices does not include shipping
Prices does not include shipping
When stated "in stock" refers to barrel blanks unless stated otherwise, not completed barrels as we are a custom shop which endeavors to provide "CUSTOM" Barrels, machined to your specs.
I Chamber some of  the barrels and leave the lengths long and the Diameters large, so I can machine your barrel to your dimensions.
Please call for lead time estimate for your custom barrel
When installed by Accurate and Reliable Rifle using my Laser alignment tool, you may be assured that you will get the best sight picture possible with the Microsight. This installation is free with purchase from Accurate and Reliable Rifle. 

Others purchased somewhere else will be charged a nominal $15.00 alignment fee.
THE FAT RAT Is the next generation of the 6MM AR-15 Platform Match calibers. This was developed by Accurate and Reliable Rifle with coorporation between Myself and four customers that wanted a little more velocity with the heavier match Sierra & Berger bullets. It is still based of the 6.5 Grendel case, but with improved chamber dimensions that allows more FPS without the pressure problems of the standard Rat and the heavier bullets
2" BLUE out of stock
6MM Rat - AR 15 Bore Guide  Only $16.99
         In Black only
AR R291
To order uppers, first email to request a quote of precisley what you want. Then after you receive the quote, call back to confirm the order with either a Credit Card or just send in a check or money order
Quotes will be valid for 30 days.
To order parts, first email to request a quote of precisley what you want. Then after you receive the quote, call back to confirm the order with either a Credit Card or just send in a check or money order
email me
quotes are good for 30 days
To order parts, first email or call to request a quote of precisley what you want. Then after you receive the quote, call back to confirm the order with either a Credit Card or send in a check or money order
email me
To order parts, first email to request a quote of precisley what you want. Then after you receive the quote, call back to confirm the order with either a Credit Card or send in a check or money order
email me

Accurate and Reliable Rifle
3935-14th st
Des Moines     IA     50313
Match Rifle Right Hand BOLT CATCH-RELEASE AR L292 only $23.99
"Lower reciever modification is required" This does not need to be done by a gunsmith, If you are handy with hand tools, you can accomplish this modification at home. But a milling mach is the simple way to acomplish it. Instructions are included.
6MM Fat Rat Dies by Whidden AR R290 ony $230.00 shipped. Comes with Micrometer seater and case gage, subtract 15% if purchaced with premium Satern or Krieger barreled upper, or 10% if purchased with just a premium barrel

BUSHING DIE ONLY, AR R292.3 $99.99
 IN stock
out of STOCK
MR SS .223 Wilson 26" 1-8, W/gas Manifold and +2" gas tube, barrel ext & Fitted bolt, Wylde comp chamber & 11 deg. target crown ONLY $355.00  PT # AR 418 W/out bolt only $300.00 (specify w/ or without bolt)

In stock
Wilson 16" Stainless Steel Light weight with threaded muzzel,1-8 twist and Wylde chamber  only $230.00 AR 450  
Stainless steel In stock
out of stock
out of stock
In stock
In stock
In stock
MR SS .223 Krieger 26" 1-8, W/gas Manifold and +2" gas tube, barrel ext & Fitted bolt, Wylde comp chamber & 11 deg. target crown ONLY $612.00  PT # AR 416 W/out bolt only $556.00 (specify w/ or without bolt)
In stock
In stock
In stock
Comming soon "HAWKEYE TACTICAL" Button rifled-HAND LAPPED 223 SR barrels
click for full view
Side charge upper W/ M4 ramps AR U127 only $274.00
these are a cold molded product, the appearance may have "pits" but they are very functional
In stock
In stock


-Brass: 6.5 Grendel brass made by Lapua. Best place to buy is

-Magazines: I have been using C-products 6.5 Grendel 10 rnd Magazines.
You can also get them at 44 Mag
or Midway USA

- Dies: Hornady custom dies, Full Length sizer die with competition seater designed for a gas gun and Grendel brass. $145.00 shipped from Accurate and Reliable Rifle
-Bore guides for 6Rat, 16.00 from Accurate and Reliable Rifle
click for printable guide
email me
All listed Loads are with Alexander Arms Lapua brass, I have not tested Hornaday brass
Hornaday brass has less CAP therefore loads need to start one full grain less
FLUTE BARREL "8  FLUTES" $110.00  FULL LENGTH 1/4" wide flutes 
Wilson SS 223 MR barrel up to 26" long, Wylde chamber & 11 Deg target crown, With fitted bolt   PT # AR 
Custom Fat Rat dies by John Whidden
                                Fat- Rat Pack
Includes 26" Krieger 1-8 twist SS barrel W/fitted bolt & WOA Competition barrel extension .750 front sight tenon, +2"gas tube, Match Rifle gas block, OAL case gage, Fat Rat bore guide, and a set of Whidden Custom Fat Rat Dies
      AR R293 Only 843.94
4 week lead time