Head Space Observations.... Can someone further explain?

[LoneWolf]

It's funny how the head spacing differs on each action. I was messing around with some barrel swaps the other day before I executed the actual plan and the HH barrel I just purchased from another member lines up the irons perfectly when head spaced on my Shilen DGR Action, but is like 30-40* off on my M10 using the same gauges and everything. Also had to retime my APA brake on my 243 barrel when I moved it from the M10 action to the Shilen Action even though the same headspace gauges were used.

Just because I'm curious can someone with a better background in the machining world explain why this is? You would think using the same gauges would line a barrel up the same way on each action you headspace a barrel on.

[Bigeclipse]

It's funny how the head spacing differs on each action. I was messing around with some barrel swaps the other day before I executed the actual plan and the HH barrel I just purchased from another member lines up the irons perfectly when head spaced on my Shilen DGR Action, but is like 30-40* off on my M10 using the same gauges and everything. Also had to retime my APA brake on my 243 barrel when I moved it from the M10 action to the Shilen Action even though the same headspace gauges were used.

Just because I'm curious can someone with a better background in the machining world explain why this is? You would think using the same gauges would line a barrel up the same way on each action you headspace a barrel on.

it would have to do with the cut threads inside the action will be different from action to action. Even a small difference will create a difference in barrel alignment. The headspace itself will be the same tolereance range rifle to rifle BUT the barrel alignment will be different which is why brakes and iron sites tend to not work so well for doing barrel swaps

[LoneWolf]

That's why I started running an APA Gen 2 Lil Bastxxx for a brake, no machining, no shims, no crush washer! It has a built in lock nut and function very well. I also did not plan on using the iron sights on the HH barrel, but thought it was interesting how one lined up and one didn't.

[Hotolds442]

I think eclipse is right, it all depends on where the thread cut starts on both the action and the barrel. Since each thread is the equivalent of .050" headspace, it doesn't take much to throw an indexed barrel off when it's placed on another action. I had thought that maybe the use of a thicker or thinner washer between the bolt face and body could change the barrel indexing, and it just may, but I haven't tried it and I'm not convinced that that's the ticket. Sourcing the washers would be difficult if not futile.

[Robinhood]

Washer won't help. The bolt head is located on the bolt body by the cross pin. The bolt head sits on the lugs. Setting the chamber back is the only simple solution outside of moving the sights.

[Steelhead]

I've seen different bolt heads changing headspace also.

[SparkyLB]

I've seen different bolt heads changing headspace also.

That makes sense. If headspace is correctly set using go/no-go gauges, and then the boltface is replaced by one that's .004" shorter, you now have borderline excessive headspace (the bolt would now close on the no-go gauge).