AR-15 bolt cutaway vid by SOTAR

[Clint]

The bolt doesn't need to move to reduce pressure on the lugs.

The forward thrust from the piston reduces lug load caused by case thrust.

The thing that nobody seems to exactly know is if the piston thrust exceeds case thrust and actually moves the bolt away from the lugs.

Let's say the bolt isn't moving forward to any degree. Is it still moving enough to relieve the pressure on the lugs making unlocking easier?

[lysander]

The bolt doesn't need to move to reduce pressure on the lugs.

The forward thrust from the piston reduces lug load caused by case thrust.

The thing that nobody seems to exactly know is if the piston thrust exceeds case thrust and actually moves the bolt away from the lugs.

In order to figure that out you need the chamber pressure vs time graph and the carrier cavity pressure vs time graph, or a reasonably good model based on these two graphs.

The Army has produced a few reports with this information in them based on the M16 and M4 gas system with M193 and M855 ammunition. Without looking up the reports and redoing the calculations, I seem to recall that in one case it is negative (the bolt thrust from the cavity pressure is higher than the residual chamber pressure, in which case the bolt is moved forward), and in other cases the bolt lug load is less than 100 pounds.

But, in contrast an AR-18 bolt lug load during unlocking is a few hundred pounds.

Here's a graph, unlocking starts around 1.75E-03 seconds, this might be that case where the bolt lug load is negative.

[Pasta123]

In order to figure that out you need the chamber pressure vs time graph and the carrier cavity pressure vs time graph, or a reasonably good model based on these two graphs.

The Army has produced a few reports with this information in them based on the M16 and M4 gas system with M193 and M855 ammunition. Without looking up the reports and redoing the calculations, I seem to recall that in one case it is negative (the bolt thrust from the cavity pressure is higher than the residual chamber pressure, in which case the bolt is moved forward), and in other cases the bolt lug load is less than 100 pounds.

But, in contrast an AR-18 bolt lug load during unlocking is a few hundred pounds.

Here's a graph, unlocking starts around 1.75E-03 seconds, this might be that case where the bolt lug load is negative.

I tried reverse image search and couldn't find the picture. Where did you find this report? Also, is this M193? 350e+6 is 50,000psi.

[lysander]

I tried reverse image search and couldn't find the picture. Where did you find this report? Also, is this M193? 350e+6 is 50,000psi.

That report is not public, so you'll have to trust me on this.

As to 50,000 psi, that is correct for calculating bolt thrust. The higher pressures are taken at the case mouth, so not representative of what's going on in the case.

[markm]

It's such an elegant and glorious design.