To all the replies that say we should not see bullet set back or jump when feeding in an AR, the Sierra article linked by 243winxb:
"During tests here at Sierra’s range, we chambered a variety of factory Match ammunition in an AR-15 rifle. This ammunition was from one of the most popular brands in use today, loaded with Sierra’s 69 grain MatchKing bullet. To conduct the test, we chambered individual rounds by inserting them into the magazines and manually releasing the bolt. We then repeated the tests by loading two rounds into the magazine, chambering and firing the first, and then extracting and measuring the second round. This eliminated any potential variation caused by the difference between a bolt that had been released from an open position (first round in the magazine) and those subsequent rounds that were chambered by the normal semi-automatic operation of the rifle. Measuring the rounds before chambering and then re-measuring after they were carefully extracted resulted in an average increase of three thousandths (0.003″) of forward bullet movement. Some individual rounds showed up to seven thousandths (0.007″) movement. Please bear in mind that these results were with factory ammunition, normally having a higher bullet pull than handloaded ammunition."
In short, Sierra saw considerable bullet jump when chambering
factory cartridges in an AR. How then can you claim we should not see bullet movement, and if bullet movement is seen, the rifle is defective? Are you claiming the rifle Sierra used for their tests was also defective? I notice that they indicated the ammo was factory Match ammunition. Their preferred proposed solution is greater neck tension. They wrote 2 to 3 thou. Unfortunately, they didn't report testing this and validating that it eliminated bullet jump in the same tests. I think it's odd that they both reported that factory ammunition "normally having a higher bullet pull than handloaded ammunition," but also that the solution is only 2 to 3 thou. I don't see handloaded ammunition likely having less than 1 to 2 thou, so if the factory ammunition already had more than that, how would 2 to 3 thou solve the problem?
My own conclusion is that it is fair play to lightly crimp the case mouth into bullets that have a crimp groove and this will contribute to preventing bullet jump. Recognizing that crimp grooves are detrimental to accuracy and that Match bullets or any bullets from which we can expect the greatest accuracy will not have a crimp groove, we will have to prevent bullet jump with neck tension because denting the bullet can only be horrible to accuracy potential. How much neck tension is needed and whether that is achieved by a bushing size, an expander size, the neck diameter prior to seating, or if it requires a certain neck hardness achieved through annealing or not annealing might have to be determined through testing of the particular brass, bullet and rifle.
I intend to test my cartridges in like fashion as Sierra:
- Measure OAL..
- Release the bolt at full force to strip a cartridge off the top of the magazine and slam it into battery.
- Eject the cartridge by hand and measure any inertial bullet jump.
- Fire a cartridge and let the action strip the next cartridge off the top of the magazine and slam it into battery at the action's speed.
- Eject the cartridge by hand and measure any inertial bullet jump.
I wrote "OAL" but I would be careful to measure the delta on each cartridge and not assume all the cartridges have the same OAL since bullets vary in length and seaters usually seat on the ogive. A bullet comparator insert could be used to minimize variance due to bullet lengths but simply accounting for each before and after measurement suffices.
