Airgun barrel choke

[TonyL]

Our JB started a thread on here a while ago, A.G., detailing his diy choking method.

Maybe he'll post a little linky up.

[thisisdonald]

I think its possible to use a big pipe cutter with the blade replaced with a hardened steel washer. you can buy these for next to nothing and is definitely worth a shot before going to all the trouble of machining and blowtorching and pressing

[lensman57]

I think its possible to use a big pipe cutter with the blade replaced with a hardened steel washer. you can buy these for next to nothing and is definitely worth a shot before going to all the trouble of machining and blowtorching and pressing

Thanks, I am considering all possibilities at the moment so this one sounds good too. So far it appears that freezing the barrel to -25C is preferable to heating the collar as the transfer of heat may alter the charactristics of barrel.

If so the contraction in diameter is about 0.03mm. I have no idea what grade of steel HW have used in the construction of the barrel but apparently the most popular steel for none military use is 4140 ( not hammer forged barrels ) which is known as ordenance steel.

A.G

[Shed tuner]

Thanks, I am considering all possibilities at the moment so this one sounds good too. So far it appears that freezing the barrel to -25C is preferable to heating the collar as the transfer of heat may alter the charactristics of barrel.

If so the contraction in diameter is about 0.03mm. I have no idea what grade of steel HW have used in the construction of the barrel but apparently the most popular steel for none military use is 4140 ( not hammer forged barrels ) which is known as ordenance steel.

A.G

I think doing that, plus have a "gentle hammer" fit bushing, is the way to go. Then machine the OD of the choke bush down to suit.

I'm planning exactly this as per the "bucketboy" method quoted on that thread a few months ago, for my "not-bad-but-not-brilliant" HW barrel, funnily enough...

[lensman57]

I think doing that, plus have a "gentle hammer" fit bushing, is the way to go. Then machine the OD of the choke bush down to suit.

I'm planning exactly this as per the "bucketboy" method quoted on that thread a few months ago, for my "not-bad-but-not-brilliant" HW barrel, funnily enough...

Thanks for the advice Jon. What would we do without the thrills of the HW barrels? Exhilirating when they work and very often they do and frustrating on the odd occasion that something is not right.

A.G

[bighit]

Thanks, I am considering all possibilities at the moment so this one sounds good too. So far it appears that freezing the barrel to -25C is preferable to heating the collar as the transfer of heat may alter the charactristics of barrel.

If so the contraction in diameter is about 0.03mm. I have no idea what grade of steel HW have used in the construction of the barrel but apparently the most popular steel for none military use is 4140 ( not hammer forged barrels ) which is known as ordenance steel.

A.G

Used to put valve guides in the freezer at work for replacing worn or broken guides in heads off engines.

[buttloaves]

Used to put valve guides in the freezer at work for replacing worn or broken guides in heads off engines.

Could the choke be polished out?

[lensman57]

Could the choke be polished out?

I haven't thought of that TBH. There maybe a possibility of altering the rifling if the choke is lapped or polished out.

A.G

[lensman57]

Used to put valve guides in the freezer at work for replacing worn or broken guides in heads off engines.

That is why I changed my mind about heating the collar and thought of putting the barrel in my brothers freezer. He has a restaurant and one of his freezers go down to -30C. If I machined a brass collar to the nominal diameter of the barrel -0.02 mm it should exert enough force on the barrel to just tighten it.

I can then test the barrel and see how the choke preforms, if not enough then the operation can be repeated with the a new collar machined to -0.04 mm and so on. I have taken the advice of Jon and the less is the way to go here.
The other indication of final size could be taken from the breech end " choke ", so the muzzel end should perhaps be at most 0.01~0.02 mm smaller.
It is all experimenting I guess.

A.G

[bighit]

That is why I changed my mind about heating the collar and thought of putting the barrel in my brothers freezer. He has a restaurant and one of his freezers go down to -30C. If I machined a brass collar to the nominal diameter of the barrel -0.02 mm it should exert enough force on the barrel to just tighten it.

I can then test the barrel and see how the choke preforms, if not enough then the operation can be repeated with the a new collar machined to -0.04 mm and so on. I have taken the advice of Jon and the less is the way to go here.
The other indication of final size could be taken from the breech end " choke ", so the muzzel end should perhaps be at most 0.01~0.02 mm smaller.
It is all experimenting I guess.

A.G

I'm not sure if this makes a difference .some guides were bronze or cast iron. It may mean different metals have different contraction when cold.

Take a measurement room temp then when frozen. Use a mike and not a verynear.

I cant remember what the size differences were frozen and room temp. Stopped doing that job 3years ago.

I do remember heating some heads (Vauxhall) and freezing the guide. We used a drift to install them. So a frozen barrel and hot band would work. Maybe constrict it too much though.
http://www.gatewaytoairguns.org/GTA/...?topic=43105.0 ideas there.

[bighit]

Some barrel info

http://www.pyramydair.com/blog/2015/...ifling-part-3/

[adie jaques]

It was on the forums, some time ago, about choking barrels. The brief synopsis is you machine an appropriate high tensile nut to a tight fit on the barrel, cut it in half with an hacksaw, put it back on the muzzle in a large vice or press, releasing a little, turning, and squeezing frequently.
I had success with a 2" square x 1/2" thick, scrap piece of fairly hard steel, bored with the appropriate hole, centrally, sawn almost in half, but held together by a couple of mil to prevent the fiddling getting it to stay in place with two separate halves. This takes a bit more slackening and turning but gets it done without finger pinching and frayed tempers. You can tell when the job is done by pushing a pellet down the bore, using a rod, thinned at the pointed end, to avoid pushing against the skirt and opening it. It usually starts tight at the breech, easy to the last 1/2" and tightens a bit again. The couple of barrels I have done had minor tight spots along the length of the barrel, but the choke was tighter. Might help to warm the barrel, a couple of hundred degrees, not enough to damage the bluing, though.