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Small bore gauge ?

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Siddley19/11/2012 15:23:20
150 forum posts
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Hi folks. I'm a gunsmith by trade and I have decided it might be fun to make my own barrels. Initially these will be for airgun use as I don't yet have the permit needed to manufacture firearms here in Spain.

I'm OK on the deep hole drilling and the rifling machine, what I haven't yet figured out is a good method of measuring the bore of the barrel. I can " slug " it with molten lead, but that's not always accurate because the lead contracts as it cools. What I would like to do is make a bore gauge, but I'm having trouble coming up with a design. It doesn't have to have much of a range as the two calibres I'll be working with are nominally .22 and .25. The accuracy would need to be in the order of .001 in

Does anyone have any bright ideas as to how I could make such a bore gauge ?

David Clark 119/11/2012 15:32:41
3357 forum posts
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Hi There

Bore gauges are readily available for these small diameters.

They are basically a split ball shape with an expander.

Just mike up the ball after adjusting to fit the bore.

I don't know if they do long ones for measuring at

a depth but I expect if you measure both ends and

they are the same the bore will be parallel.

regards David

JasonB19/11/2012 15:34:08
21431 forum posts
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Talking to yourself again David???


Edit Ah added the question now

Edited By JasonB on 19/11/2012 15:34:57

David Clark 119/11/2012 15:46:52
3357 forum posts
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10 articles

Hi Jason

I had to aprove the first post which I did straight after posting.

regards David

Siddley19/11/2012 15:55:44
150 forum posts
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Thanks David ( I understand the moderating, I know what a pestilence spammers can be on forums )

I had seen the ball ended bore gauges but didn't really know if they were up to much. I'll have to get some sent over from the UK with my next 'care parcel' of engineering goodies then.

David Clark 119/11/2012 15:59:35
3357 forum posts
112 photos
10 articles

Hi Siddley

They are very good.

You can get good quality Moore and Wright ones from Ebay.

A set is deasr but you would only need a couple of odd ones.

regards David

Edited By David Clark 1 on 19/11/2012 15:59:51

Michael Gilligan19/11/2012 16:25:32
18920 forum posts
940 photos
Posted by Siddley on 19/11/2012 15:23:20:

I can " slug " it with molten lead, but that's not always accurate because the lead contracts as it cools.


Welcome, Siddley

If I recall correctly; the Bismuth based Low Melting Point alloys [Wood's Metal and similar] actually expand very slightly on solidification. ... Assuming that you can still push the "slug" out; this might make them useful.



Useful reference document found here

See Table 9


Edited By Michael Gilligan on 19/11/2012 16:48:04

Siddley19/11/2012 16:58:45
150 forum posts
1 photos

Thanks for jogging my memory Michael, you reminded me that there is a product sold in the USA called 'Cerrosafe' which is intended for making castings of the chamber in a firearm barrel, so it can be more easily measured.

It's a proprietary formula, but I bet it's very similar to Woods metal...I'll have to see if I can get some

I think a Moore and Wright gauge from Ebay will be my first port of call though

Edited By Siddley on 19/11/2012 16:59:20

Clive Hartland19/11/2012 19:50:30
2724 forum posts
40 photos

Siddley, having been a mould maker for various cal. I only ever pushed a lead slug through the bore, mostly 9mm or .357 and occasionally .44, I have slugged a .22 this way and its plenty accurate except if you have an odd number of lands.

I once did a black powder slug of .567 in a 5 groove ogive rifled barrel made by Joseph Harcombe and it worked fine. I slugged the barrel with a cold lead slug and turned a metal gauge until the slug entered and that was the diia. of the mould I made.

If you need to measure bore dia. then the drilled /reamed bore plus the depth of rifling will be fine.

I remember that the .22 has a wide variation of tolerances and if its for and air weapon then make a swaging die so that the slugs/pellets fit the rifled barrel.

Its quite common here in the UK to buy pellets to fit the air rifle barrel that vary a thou or two. Then for consistency to push the pellets through a swage to make them truly round and the same dia.

Measuring bores is usually done with a, 'Go', 'No Go' set of gauges on a rod that can be passed through the bore when you can feel any tightness or even looseness.


Edited By Clive Hartland on 19/11/2012 19:51:52

Siddley19/11/2012 21:53:10
150 forum posts
1 photos

What a helpful and interesting post Clive, thanks a lot.

Just so you know - I was a licenced gunsmith ( RFD ) in the UK before emigrating to Spain, but never got into the finer points of bore versus bullet size. Unless you are doing work for the muzzleloading guys it doesn't really come up.

I want to experiment with different bore sizes, tapered bores and microgroove rifling. Airguns seem to be extremely fussy about pellet sizes and I'd like to try and find out why. I also have a commercial reason as the ( now prohibited in the UK ) Brocock air cartridge guns are VERY popular here. I'm intending to make a few BACS rifles and pistols from scratch, something I did in the UK before they were effectively banned.

I'd also like to do some model engineering, but I have been saying that now for at least 15 years and all I have to show for it are good intentions. Maybe I'll just keep reading about it instead

Clive Hartland19/11/2012 22:29:56
2724 forum posts
40 photos

Siddley, how are you doing the barrels, are you buying in or making them and rifling yourself?

The quality of the bore is paramount for small calibers and more so for air weapons with limited pressure to propel the missile. In a center fire rifle they are mostly swaged now as it gives a high Rockwell hardness number. The swages are soldered to rods and then hydraulically pulled through a prepared bore with a bearing to allow the barrel blank to turn as the rifling swage goes down. High tech lubricants are used to ease the friction of the swaging. This gives a very smooth bore.

Cut rifling blanks are very expensive at about £175 per.

For air weapons the pellet weight and balance is very important, in a particular barrel one pellet is fine but put up the weight and have a pointed head and it does not go at all.

One of the latest barrels supplied here in the UK for an air gun is smooth bore up until the last 2 or 3 inches and then is finely rifled, the idea is to get the pellet moving and then engage the rifling.

Dry lubrication of airgun pellets is also a big thing now, dip and dry and off they go.

A similar idea with CF rifles where the Leed is made longer so that the bullet can jump into the rifling. Nominally the bullet should just engage the rifling when the bolt is closed but not move the bullet. I know of rifles that have a leed of at least 3 bullet dia.

Out of interest one of the most accurate .303 rifles I ever used only had two grooves in the barrel, it was made in Canada during the war years.


David Littlewood19/11/2012 23:02:16
533 forum posts


Just a minor pedantic correction: it is still legal to own self-contained air cartridge weapons (SCACs, aka "Brococks" ) in the UK provided that you owned them before the ban came in and you got them put on a firearm certificate before then. You are not allowed to sell them or give them away (you can't even leave them to your descendants in your will). I'm not sure what can be done with them (legally, I mean) though I can't immediately see anything in the legislation that stops you exporting them for sale abroad....


The bloodly smiley appeared again! I got rid of it by putting a space between the " and the ).

Edited By David Littlewood on 19/11/2012 23:03:38

Siddley20/11/2012 01:04:12
150 forum posts
1 photos

Clive - I'm going to make the barrels myself. The idea of a smoothbore terminated with a rifled extension dates back to WW-2 I think ? I'm sure there was an anti-tank gun which used that concept. Have you heard of the UK airgun company called Theoben ? the 'Ben' in the Theoben name is a friend of a friend. I don't claim to know him, I think I have spoken to him maybe twice on the phone. A very talented guy ( in my opinion at least ) - last I heard he had left the company and was making microgroove barrels. I'm quite interested in that technology.

David - Never apologise for being pedantic when it comes to firearms legislation. It's always good to stay out of jail ( even if you are a biker who probably has more tattoos than the inmates, like me   

I actually had an L42A1 air cartridge replica on my FAC ( I had an FAC as well as an RFD, it's not the done thing to mix business with pleasure )  which I built from one of the old Saxby-Palmer Enfields - the barrel was from a shot out target rifle, bored out to take a .22 liner and still retained about half an inch of the original rifling at the muzzle. It really looked the part but wasn't much practical use.

I've got quite a collection of Brocock guns here. Two of them are revolvers which started life as Weihrauch Arminius in .38 special, another two are Uberti single action Colt clones, there are two others which are monkey metal junk, a Predator rifle, the 'Fox', and a Uberti 1866 Yellowboy.

I'm tempted to design a new cartridge which is capable of supplying more muzzle energy - the legal limit for airguns in Spain is 18ftlbs and the BACS gave something like 10ftlbs out of a rifle barrel.

I hope talking about ( legally owned ) guns is OK on this forum ( I don't intend to make a career out of it ) - if not then please will a moderator tell me to shut up ? smiley

Edited By Siddley on 20/11/2012 01:05:12

JohnF20/11/2012 01:18:47
1099 forum posts
183 photos

Siddley, more or less all been said, but slugging is the easy way to go and possibly the most accurate, its difficult to measure over the lands of rifling and almost impossible to measure the groove dia without specialist tools on small calibres.

Cerrosafe, Cerrobend are forms of Wood's Metal both available in the UK -- last I bought was from Frys metals about 30 years ago.

Good for measuring firearm chambers or bores but be sure to lubricate first otherwise it will stick to the parent metal -- just like tinning when soldering. Also good to lube the barrel when slugging.

Air gun ammo is sold in two sizes 5.5 & 5.56mm most continenetal ammo is 5.5 and UK / USA is 5.56


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