SA80 Replacement on the distant horizon ?

(...) A couple of weeks ago, when I was still planning on doing some slightly complicated stuff involving attaching cleaning rods to cases and watching/feeling them to prove it, (...)
I assume that you had something like the following in mind.
  • Tap a thread into the base of an empty cartridge case.
  • Clamp the rifle muzzle down on the side of a workbench.
  • Chamber the cartridge with the cleaning rod attached, or screw the cleaning rod into it from the muzzle end.
  • Use a bolt without a bolt carrier if possible and rotate it to the locked position. It would be preferable to use a bolt without bolt carrier as this would give you finer and gentler control over the bolt when unlocking it.
  • Some sort of bushing or sleeve for the cleaning rod at the muzzle (in the form of a muzzle cap with a hole) would be preferable in order to minimize measurement errors.
  • Clamp a dial (or digital) indicator over the muzzle and zero it on the end of the protruding rod.
  • Slowly rotate the bolt to the unlocked position but do not withdraw it.
  • Measure the amount of movement of the dial indicator.
  • Rotate the bolt back into the locked position to confirm that it goes back to zero.
  • Repeat several times

If there is a significant amount of movement the dial indicator should pick it up.

This set up should be adaptable to a variety of different rifles and cartridges.
 
I assume that you had something like the following in mind.
  • Tap a thread into the base of an empty cartridge case.
  • Clamp the rifle muzzle down on the side of a workbench.
  • Chamber the cartridge with the cleaning rod attached, or screw the cleaning rod into it from the muzzle end.
  • Use a bolt without a bolt carrier if possible and rotate it to the locked position. It would be preferable to use a bolt without bolt carrier as this would give you finer and gentler control over the bolt when unlocking it.
  • Some sort of bushing or sleeve for the cleaning rod at the muzzle (in the form of a muzzle cap with a hole) would be preferable in order to minimize measurement errors.
  • Clamp a dial (or digital) indicator over the muzzle and zero it on the end of the protruding rod.
  • Slowly rotate the bolt to the unlocked position but do not withdraw it.
  • Measure the amount of movement of the dial indicator.
  • Rotate the bolt back into the locked position to confirm that it goes back to zero.
  • Repeat several times

If there is a significant amount of movement the dial indicator should pick it up.

This set up should be adaptable to a variety of different rifles and cartridges.
It was a bit more rustic than that, with a spud threaded onto a rod and crimped / superglued into the neck of a case, deformed a little to stick in the chamber nicely. I'd leave the carrier on cos otherwise manipulating the bolts would be a pest, and also the fudds will claim that the carrier's doing some mechanically-impossible magic.
 

Themanwho

LE
Book Reviewer
It was a bit more rustic than that, with a spud threaded onto a rod and crimped / superglued into the neck of a case, deformed a little to stick in the chamber nicely. I'd leave the carrier on cos otherwise manipulating the bolts would be a pest, and also the fudds will claim that the carrier's doing some mechanically-impossible magic.
Very rustic!

I'd suggest an easier and less tuber-based method would be to smear a thin coat of engineers blue on the base of live rounds, and fire them (this would also allow the expansion of the case into the chamber and into the bolt face, which dry cycling - even with a spud - wouldn't). Witness marks on the blue would show rotation marks on the bolt head if the cartridge didn't turn with the bolt.

Simples.
 

CC_TA

LE
I assume that you had something like the following in mind.
  • Tap a thread into the base of an empty cartridge case.
  • Clamp the rifle muzzle down on the side of a workbench.
  • Chamber the cartridge with the cleaning rod attached, or screw the cleaning rod into it from the muzzle end.
  • Use a bolt without a bolt carrier if possible and rotate it to the locked position. It would be preferable to use a bolt without bolt carrier as this would give you finer and gentler control over the bolt when unlocking it.
  • Some sort of bushing or sleeve for the cleaning rod at the muzzle (in the form of a muzzle cap with a hole) would be preferable in order to minimize measurement errors.
  • Clamp a dial (or digital) indicator over the muzzle and zero it on the end of the protruding rod.
  • Slowly rotate the bolt to the unlocked position but do not withdraw it.
  • Measure the amount of movement of the dial indicator.
  • Rotate the bolt back into the locked position to confirm that it goes back to zero.
  • Repeat several times

If there is a significant amount of movement the dial indicator should pick it up.

This set up should be adaptable to a variety of different rifles and cartridges.
Thought about trying a decent chamber laser boresight? (As long as you know the original error by rotating through 90, 180 and 270 degrees to confirm it might be useful.)

  • Insert whilst bench mounted
  • Red dot on sighting screen at known distance
  • Lock bolt
  • Measure difference

BS223-2.jpg
 
Thought about trying a decent chamber laser boresight? (As long as you know the original error by rotating through 90, 180 and 270 degrees to confirm it might be useful.)

  • Insert whilst bench mounted
  • Red dot on sighting screen at known distance
  • Lock bolt
  • Measure difference

View attachment 634343
We're really talking about measuring linear movement along the axis of the bore. Whether or not there is any rotary movement is just a distraction.
 
It was a bit more rustic than that, with a spud threaded onto a rod and crimped / superglued into the neck of a case, deformed a little to stick in the chamber nicely. I'd leave the carrier on cos otherwise manipulating the bolts would be a pest, and also the fudds will claim that the carrier's doing some mechanically-impossible magic.
The Chap has got a nice little workshop, doesn't he? He may have the kit required. If not, you could see if you could borrow it. He could certainly make you a bushing and other bits (e.g. barrel clamp for the indicator) on his lath.

Tapping was a suggestion, but if that is too hard then epoxying just about anything with the right thread into the cartridge case would do the same job.

I've seen off brand dial (and digital) indicator with magnetic base kits for around the $50 mark. You won't get a Mitutoyo for that sort of price (or anything close to it), but you may get something that is good enough. Either a plunger indicator or test indicator would probably be good enough.

I'm not of course suggesting that you need to do any of this, but if you are looking for suggestions then this sort of set up should be good enough to produce results that are repeatable, reproducible, and inarguable.
 
[
At a rough guess, it requires transmitting the load on the back face through the receiver, rather than straight back into the barrel/barrel extension like a rotary locking bolt. Other things being equal a load bearing receiver is heavier and more expensive than a non-load bearing receiver.

And that receiver would have to accommodate the fact that 5.56 has a chamber pressure about 50% higher than 7.62x51 ( 78 versus 52 kpsi ). So any design locking into the receiver has to be stronger for the smaller round!

FN tried tilting-block 5.56 with the CAL but quickly moved on to the AK-derived FNC.
 
[

And that receiver would have to accommodate the fact that 5.56 has a chamber pressure about 50% higher than 7.62x51 ( 78 versus 52 kpsi ). So any design locking into the receiver has to be stronger for the smaller round!

FN tried tilting-block 5.56 with the CAL but quickly moved on to the AK-derived FNC.
It's a higher pressure, but it's acting over a much smaller surface area ;)
 

Themanwho

LE
Book Reviewer
The Chap has got a nice little workshop, doesn't he? He may have the kit required. If not, you could see if you could borrow it. He could certainly make you a bushing and other bits (e.g. barrel clamp for the indicator) on his lath.

Tapping was a suggestion, but if that is too hard then epoxying just about anything with the right thread into the cartridge case would do the same job.

I've seen off brand dial (and digital) indicator with magnetic base kits for around the $50 mark. You won't get a Mitutoyo for that sort of price (or anything close to it), but you may get something that is good enough. Either a plunger indicator or test indicator would probably be good enough.

I'm not of course suggesting that you need to do any of this, but if you are looking for suggestions then this sort of set up should be good enough to produce results that are repeatable, reproducible, and inarguable.

Unless a round is fired and the case expands in the chamber and onto the bolt face you won't be able to determine whether a form of primary extraction is given by the bolt's unlocking rotation in the cycle of operation. You'll just be cycling the action with a drill round / empty case / laser boresight / potato.

For my money Stoaty's probably right that it doesn't but for conclusive proof, the action would have to be operated by firing to establish what happens; unless anyone can think of a way to accurately replicate the brass expansion caused by firing .

Any case rotation with the bolt could be measured either by engineer's blue on the cartridge base as described previously, or marking the external diameter of the cartridge case as below, and recording its location on extraction using a fast frame camera if you have one (for a semi auto rifle, which is what I'd suggest you need to use so that the extraction process works as designed and the heat generated by firing is a factor, as well as any effect from gas operation on it.

1643195764033.png


The engineer's blue option is simpler and doesn't require as much faffage.
 
Unless a round is fired and the case expands in the chamber and onto the bolt face you won't be able to determine whether a form of primary extraction is given by the bolt's unlocking rotation in the cycle of operation. You'll just be cycling the action with a drill round / empty case / laser boresight / potato.

For my money Stoaty's probably right that it doesn't but for conclusive proof, the action would have to be operated by firing to establish what happens; unless anyone can think of a way to accurately replicate the brass expansion caused by firing .

Any case rotation with the bolt could be measured either by engineer's blue on the cartridge base as described previously, or marking the external diameter of the cartridge case as below, and recording its location on extraction using a fast frame camera if you have one (for a semi auto rifle, which is what I'd suggest you need to use so that the extraction process works as designed and the heat generated by firing is a factor, as well as any effect from gas operation on it.

View attachment 634462

The engineer's blue option is simpler and doesn't require as much faffage.
If you deform the case and force-chamber it, you've got bolt face pressure too. That's what I did in the earlier videos.
 

Themanwho

LE
Book Reviewer
If you deform the case and force-chamber it, you've got bolt face pressure too. That's what I did in the earlier videos.
Good stuff, but I'd contend that force-chambering runs the risk of further deformation potentially in a different direction, and unless the cartridge is directly aligned to its original location in the chamber when fired its contact with both chamber and bolt may not be exactly as when originally fired, rendering it an imperfect model of the extraction.

Nit picking git, I know.
 

rampant

LE
Kit Reviewer
Unless a round is fired and the case expands in the chamber and onto the bolt face you won't be able to determine whether a form of primary extraction is given by the bolt's unlocking rotation in the cycle of operation. You'll just be cycling the action with a drill round / empty case / laser boresight / potato.

For my money Stoaty's probably right that it doesn't but for conclusive proof, the action would have to be operated by firing to establish what happens; unless anyone can think of a way to accurately replicate the brass expansion caused by firing .

Any case rotation with the bolt could be measured either by engineer's blue on the cartridge base as described previously, or marking the external diameter of the cartridge case as below, and recording its location on extraction using a fast frame camera if you have one (for a semi auto rifle, which is what I'd suggest you need to use so that the extraction process works as designed and the heat generated by firing is a factor, as well as any effect from gas operation on it.

View attachment 634462

The engineer's blue option is simpler and doesn't require as much faffage.

Compressed air might work, but the set up would be even more of an arse than than terminal's suggestions, and things can go wrong fast.

On the other hand, allows for repeatable testing with controlled pressures
 
I’d be cautious about any system running fluid pressure enough to replicate firing pressures. Lots of stored energy.
If you filled the cartridge with PU and pushed on that you’d solve the stored energy problem but you’d need to react the force on that through the rifle, you wouldn’t get any thermal effect and at the end a cartridge full of PU probably doesn’t behave like an empty cartridge.
 

jrwlynch

LE
Book Reviewer
I’d be cautious about any system running fluid pressure enough to replicate firing pressures. Lots of stored energy.

We got round that, trialling one system, by running it with water for the test & proving phases, before going to the compressed air it was designed for during its endurance trial. If we found a leak, water was (a) much easier to spot spraying out and trace to a source, and (b) had a lot less energy to kill careless spanner-monkeys like me with.
 
We got round that, trialling one system, by running it with water for the test & proving phases, before going to the compressed air it was designed for during its endurance trial. If we found a leak, water was (a) much easier to spot spraying out and trace to a source, and (b) had a lot less energy to kill careless spanner-monkeys like me with.
At 500MPa liquid would cut you in half.
In al fairness PU is probably going to be liquid at that point too.
 

jrwlynch

LE
Book Reviewer
At 500MPa liquid would cut you in half.

Well, we were "only" working at 400psi (showing how long ago it was), or about 3MPA. Not sure how you get 500MPa in a home workshop (and what the neighbours might have to say - very thick walls and a very thin roof required?)

Still made a lot of sense to use water rather than air for pressurising the rig during the "build, try, dismantle, fiddle, reassemble" phase...
 
Still made a lot of sense to use water rather than air for pressurising the rig during the "build, try, dismantle, fiddle, reassemble" phase...
Very much so. incompressible fluid stores much less energy than compressible ones.
Not sure how you get 500MPa in a home workshop
1.2 tonne on a rod acting as a piston? column bucking might be a thing.
 
Unless a round is fired and the case expands in the chamber and onto the bolt face you won't be able to determine whether a form of primary extraction is given by the bolt's unlocking rotation in the cycle of operation. You'll just be cycling the action with a drill round / empty case / laser boresight / potato.

For my money Stoaty's probably right that it doesn't but for conclusive proof, the action would have to be operated by firing to establish what happens; unless anyone can think of a way to accurately replicate the brass expansion caused by firing .

Any case rotation with the bolt could be measured either by engineer's blue on the cartridge base as described previously, or marking the external diameter of the cartridge case as below, and recording its location on extraction using a fast frame camera if you have one (for a semi auto rifle, which is what I'd suggest you need to use so that the extraction process works as designed and the heat generated by firing is a factor, as well as any effect from gas operation on it.

View attachment 634462

The engineer's blue option is simpler and doesn't require as much faffage.
In this instance we are looking for whether the extractor pulls back on the cartridge as the bolt rotates to the unlocked position. If it doesn't move the cartridge during that phase of the process then whether or not it can pull a stuck cartridge loose is irrelevant.

We're talking about a simple experiment to test one simple thing. If the cartridge doesn't move during that experiment then whether or not the cartridge was expanded by firing would be irrelevant.

If you really think it makes a difference then do the experiment I outlined using a fresh cartridge case and one from the same lot that had been fired once already and see if there is any difference.

I also don't agree that machinist's bluing will show the evidence unequivocally as you are making indirect inferences rather than direct measurements. Inferences may be wrong as they are based on assumptions and what is wanted in this case is visual proof on video of the thing in question actually happening or not happening.
 
Interestingly the latest video up from Gun Jesus strongly infers that the L119A2 is not that popular vs the L119A1 so I do think Caracal/Haenal amongst others might have a good chance at the Ranger contract:

 

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