What ever happened to the 6.5MPC and how would it stack up to the Grendel?

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  • What ever happened to the 6.5MPC and how would it stack up to the Grendel?

    I haven't heard anything about the MPC in something like 3 years now, so I'm assuming that it's dead and gone, but conceptually I like it. My primary question would be if anyone knows anything about differences in case capacity versus the 6.5G?

    Has anyone ever even seen the MPC in the real world?

    Cheers,
    Colorado
  • bwaites
    Moderator
    • Mar 2011
    • 4445

    #2
    Well, basically ballistics! The case is too small to actually allow enough propellant to use reasonably heavy bullets.

    When you got to the 6.5 bullets at decent weights, (where you could get better ballistic coefficients) say 120 grains or so, you ended up with the ballistics of a 7.62x39 round. So, you basically ended up with a 60 year old ballistic cartridge.

    The Grendel kicks those same projectiles along several hundred feet faster.

    I know that there were a few rifles floating around for a while, but haven't heard much in at least 2 years.

    Comment

    • sneaky one
      Chieftain
      • Mar 2011
      • 3077

      #3
      Wilson has their version of it out now, 6.5 x 40 WT. Change out the 5.56 bbl. and it's go time. Stats. look about the same as GRR. in 90-100 grn. class.

      Comment

      • cornholio1

        #4
        I have one. Its actually pretty good. I am getting similar results as Grendel, but Grendel is ultimately a little better. I thinking about selling the MPC upper tho. I just don't have time for it. You need to trim the 223 cases quite a bit.

        Comment


        • #5
          I can see the diminished case capacity being a significant handicap when it comes to the ability to launch heavier and longer projectiles at sufficient velocities.

          So next question: Do steel cases handle higher pressures better than brass? I know steel has a modulus of 200GPa, and brass has a modulus of 100-125 GPa, so conceivably one could use a combination of slow and fast burning powders to cook up some real velocity out of a smaller case. I know from experience that steel is not the most maleable material to work with, making reloading and manufacturing a PITA, but it seems like it could offer some benefits.

          Disclaimer, I am a Mechanical Engineer, and underemployed, so I haven't had any experience with reloading due to budget constraints. I also have next to zero knowledge of propellents, so I'm not sure if my though experiment would even be valid... I just have a theoretical knowledge of materials and a passion for firearms and a desire to make them better.

          Thanks for all of your insight.

          Regards,
          Colorado

          Comment

          • mtn_shooter

            #6
            Mixing different powders is 99.999% (read 100%) of the time a no-no.

            Comment

            • DirectDrive

              #7
              Originally posted by mtn_shooter View Post
              Mixing different powders is 99.999% (read 100%) of the time a no-no.
              To do this you have to be (1) foolish and (2) lucky that the shrapnel blows away from your face and hands.
              No kidding whatsoever.

              A steel case isn't going to magically increase the designed pressure capacity of your rifle's bolt and chamber. Pressure is pressure, doesn't matter if it's in a balloon, brass or steel, the locked chamber has to contain it.
              And who wants to monkey with reloading steel cases....I would never entertain that idea unless I was in some extreme situation.

              Comment

              • Clod Stomper

                #8
                Originally posted by sneaky one View Post
                Wilson has their version of it out now, 6.5 x 40 WT. Change out the 5.56 bbl. and it's go time. Stats. look about the same as GRR. in 90-100 grn. class.
                That's interesting. I have read about the 7.62x40, but a search on the 6.5x40 turns up nothing for me. I'm not doubting, but I would love to read more about it.

                To the OP, check out this wildcat, the 6.5 PCC. http://www.ar15armory.com/forums/65m...ar-t95187.html

                The developer apparently tried the 6.5 MPC and wasn't satisfied, so he developed his own with a longer case. Looks like it's approaching the Grendel velocities with lighter bullets. For the heavier bullets, I'll stick with my Grendel.

                Will

                ETA: Wait, is this it? http://www.beyond556.com/bboard/arch...p/t-4870.html?
                Last edited by Guest; 07-15-2011, 09:49 PM. Reason: PCC. Not PPC. Oops.

                Comment


                • #9
                  Originally posted by Colorado View Post
                  ...Do steel cases handle higher pressures better than brass?...
                  I can't give a definitive answer on the difference in potential of the two materials to handle pressures in the Grendel --

                  One way to think of the environment seen by the cartridge case is to understand the role of the cartridge in the chamber. Prior to insertion, the cartridge is merely a sack to hold the bullet, primer, and powder. On ignition of the powder, the next function is to obdurate, or seal, the breech to prevent gas leakage. The case needs to at least elastically deform to accomplish this.

                  The higher pressures used in some cartridges do indeed cause plastic flow of the brass. Reloaders frequently take advantage of this by measuring the change in base diameter resulting from the shot. The deformation is compared with that seen with factory loads -- comparable expansion is viewed by many to indicate a reasonably safe load. Go seriously high with the pressure and the case will fail by extrusion through the largish opening at the bolt-breech interface. A higher strength material would indeed delay the onset of the plastic flow.

                  The lugs of the bolt are designed to take up the thrust of the cartrige. Most, but not all, of the time the thrust is reduced by the friction between the breech wall and the cartridge. Remember, 50 ksi+ (345 mPa) pressures generate a lot of frictional contact force even with low coefficients of friction. The problem is that we can't count on the case to always contain the thrust. Oiled cartridges, the occasional soft brass, incipient separation at the base, etc., all allow an increase in bolt thrust. Hence the bolt must be designed to support the full thrust generated by the product of the pressure and the base area measured inside the cartridge.

                  A subtle issue with cartridges like the Grendel is that the .445" O. D. cartridge base diameter is larger than the 0.378" O. D. base diameter of the .223 Remington (5.56NATO) for which AR-15 bolts were designed to handle. The larger resulting inside diameter means a fair bit of additional bolt thrust at any pressure. This is why you rarely see published pressures for the Grendel exceeding about 52,000 psi.

                  This pressure is low enough that the difference in strength between steel and brass is moot.

                  Cheers!

                  PS I know the discussion only raises more questions ... that's part of the fun of this forum!

                  Comment

                  • StoneTower

                    #10
                    There is also the issue that a steel case is less ductile (plastic) than a brass case and therefore will tend to grip the chamber less than a brass case. It is common to see sooted cases when shooting steel cases. As said above if the case does not grip the chamber wall, the bolt is under much more stress when the cartridge is fired even if the case itself is stronger and can take more pressure.

                    The smaller case head of the 6.5MPC should be able to take more pressure than the 6.5 grendel. If you had a rebated case head on a Grendel, you might be able to up the pressure in the AR15 platform.
                    Last edited by Guest; 07-15-2011, 04:10 PM.

                    Comment


                    • #11
                      The reason for the soot is more likely that the elastic modulus of steel is almost twice that of brass. This means it takes more pressure to get the seal. In effect, it takes a little longer to get the seal, and this gives the gases a better chance to work around the lip and back up the case.

                      Rebating the rim won't affect bolt thrust because bolt thrust is set by the pressure and the area on theinside of the case head.

                      Comment


                      • #12
                        Though wouldn't the rebated rim allow more steel around the case head, like the 5.56, making the bolt stronger?

                        Comment


                        • #13
                          Originally posted by z06man View Post
                          Though wouldn't the rebated rim allow more steel around the case head, like the 5.56, making the bolt stronger?
                          The problem is that, as long as one uses a bolt that can swap in and out of a standard upper, you're more or less stuck with the lugs you have. There's enough meat there to safely and reliably support the push-back the 5.56 NATO gives.

                          When one goes to a cartridge with a larger inside diameter at the base, the area available for the gases to push on increases. Hence the rearward thrust increases even when you use the pressures considered safe in the .223 Remington.

                          My guess is that the folks designing and making uppers for cartridges like the WSSM series had to build a whole new upper that makes room for a larger diameter bolt-head and the beefier lugs. So, yes, there is a work-around, but it makes for a more expensive upgrade.

                          Comment

                          • StoneTower

                            #14
                            The WSSM uses a standard upper that has been modified. I think the carrier is also modified to take an AR-10 bolt but the carrier still has to slide back into a standard buffer tube.

                            Comment


                            • #15
                              Thanks -- didn't know the details.

                              Follow up question -- how costly is that mod if applied to an M4?

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