This is a discussion on Groved or Slotted Pistons within the Art Luppino forums, part of the Gun Professionals category; Ditto to Brown, Sarge, and others.
We (Army) cut those slots by hand. It reduced the pressures caused by the heavier bullets for a smoother ...
Are you talking about this jewel? This priceless unit is the enabler to the best shooting M1A that came from your shop... It's for sale and you have the exclusive right to buy it.
Bamban, your picture throws some confusion for me since the groove does not extend to the tail of piston. In your example the groove provides no new path for the initial pressure spike. In fact it would allow MORE gas to flow into the pressure side of the piston as it moved past the barrel port (assuming the barrel pressure was still higher) More likely the barrel pressure has dropped by the time the piston moves at which point the gas would follow the groove path back out to the barrel until equalized.
Most of the reasons for a slotted piston have been expressed, thanks to all of you. If this were a contest, first place would go sadlack. Don't understand how we all missed it, maybe we are too close .. When all else fails,,, tag a National Match name to it, than stand aside to allow the mass of Lemming's past.
The best reason for a slotting piston seems to be reducing presssure, the best way to reduce pressure would be to make a mod to slow the Dwell time, than tag that mod to read, NM Dwell... Art
In reality it's about volume. The amount of gas entering the gas cylinder is volume controlled by the gas cut off when the piston moves out of alignment with the gas port. Shooting heavier bullets forces pressure up at the gas port and increases it's volume before it's cut off. The groove serves to relieve any excess pressure in the system. Keep in mind that the gas continues to expand even after cut off, forcing the piston back. if more volume is injected it exerts more force on the operating rod and bolt, resulting in higher velocity, which can damage things. I never found it to be a problem with 168 and 173 grain match ammo. Remember this ammo is loaded to 2550 FPS velocity which is quite moderate. The issue with the 175 M118LR is that it was loaded for use in the M40 and M24 rather than the M14 (ask Sierra). I'm not sure of it's impact on the M110 which is gas operated, but uses a different gas system. I've tried using slotted pistons in some M14 type rifles I've built, but they often cause short cycling with M118 and M852.
Interesting thing about these modified pistons, as far as I know nobody seems to have the reason for the Mod.. Like everyone else, I have heard a fair number of reasons for the grove/slot, but if you question the party advancing their explanation they have no facts to offer. Of coures they charge you more for one.. Toss in a platted pistion and you have made someone happy..
The best explanation came from a retired Marine that spend his career in the RTE shop. He recalls their first apperance, they cut the slots my hand, the grove wondered all over the place, much like the one Banbam owns, He retrieved out of my trash bucket.
The Mariine's junked the mod to the piston, claiming it did nothing to improve the M14's. If you own and run a slotted piston and like it, who needs an explanation. Personally I believe it is an ongoing exercise in expiration.. Cranky Art
I was always under the impression that it was Sgt. Donovan that came up with the groove and it was his intention too shoot 180grn+ bullets with the intension of not beating the rifle or its bedding too death on long range rifles? This would have been pre medium and heavy weight barrels. The unintended side effect was that the small skinny barrels flexed less so they had less up pitch at the muzzle but the heavy bullets still beat the rifle and bedding.
Originally Posted by Old Sarge
Who am I to disagree with guys like Cranky Art and Ted Brown, but it has been my understanding that the groove was intended to relieve the higher port pressures when using heavier bullets, slower powders, etc. and was meant to ease the pounding on the gas system. Much like the Schuester gas plug. Not necessarily improve accuracy... Correct me if I'm wrong.
I'm with you almost.... I thought it was too continuously vent the gas system as it cycled so there was less clang and bang when slower powders were used with heavy bullets....