7.62mm ASHOT rifle - Part 3: Chamber

Previous Chapter - Part 2: Locking mechanism

 

Chamber made from 12/20mm ID/OD steel tube. Metal warp under the heat from welding, causing the bore to shrinks and cartridges can no longer seated. It's the nature of metal expansion, there's no avoiding that. But where there's a hole, there's a way.

An adjustable reamer can be use to bore out the chamber to spec again (for this particular reamer, the size can be adjust from 11.5 to 13mm in diameter).

This tool is somewhat crude, it's hard to set the reamer to the correct size, even when measured with a caliber.

PVC cartridges are very sensitive to chamber size. A bit oversize and the case will crack. Undersized - extraction gonna be a bitch

case rupture due to oversized chamber. The size difference was only 0.1mm   

Case ruptured causing hot gas to escaped from breech face

  

Split case leave lines of sooth on the chamber wall

Reaming a usable chamber is a matter of trial and error, so I test it on a short tube first, making sure the size is correctly set before running on the full length chamber.
You can even try to ream a plastic tube, it's alot easier and wear out your tool less

 

Start small, add tiny increment as you progress. This is the most efficient way to dial in your chamber size. Rushing it and you risk make an oversized hole. You'll have to start all over again.

For me, the metal shrunk ~0.1mm  (the resolution on my caliper was 0.05mm). Took me 3 passes with tiny incremental to dial it in. One small bit at a time. 

The case must not wobble when seated. A small resistant should be felt when loading the round in, this indicate good contact between chamber and casing. 

Once I got my reamer dial in, I start working on the real chamber.

 

The direction from which your reamer enters is also important. As I mentioned earlier, this is a crude tool, imperfections are expected, especially if you do it by hand like me (might works better on a lathe). 

The first few centimeters of reaming, the tool might canted just a tiny little bit, enough to not make the hole not perfectly round. This imperfection will cause head separation, case split, following by gas escape from breech face.

To mitigate this, start reaming from the throat, where the barrel and chamber meet. A little oversized at this area is not too much of a problem, washer can be insert between chamber and barrel to form a seal if there's any gas leak around that area.

As we bore deeper and deeper, the holes forms a pilot that keep the shank straight. When the reamer reachs the other end, we already have sufficient support, result in a more uniform, round exit hole.

Below are spent cartridges fired from 2 different chambers. One was reamed from the breech face, the other from the throat

 

As you can see, the direction from which the reamer enters affect where the case separation occur.

A case neck separation doesn't leak too much gas from breech face, and having an intact case head along with its 2 lead wire to extract is far more desirable then having a half broken case stuck inside the chamber.

To test whether my chamber is correctly sized. I filled up a PVC case with black powder, capped both ends with hot glue. Then load it into the chamber, and trigger a detonation under the water.

 

 

The detonation putting stress onto the case wall against the chamber. I then inspect the case for cracks and separation (especially at case head area). If it's intact, that's a win.

 

Next chapter - Part 4: Rifled barrel