BREECH PLUG

[wapiti22]

I'm working on my first build. Recently picked up a new GR Douglas barrel in 45 cal. Has a breech plug that is not fully seated. I ordered a different plug to install.  The depth of the shoulder in the barrel is .394. The thread length on the new plug is .625.  My question is this, is .394 deep enough for the plug to seal and be safe?  I don't own a lathe or have ready access to one so any work will be by hand. I think it wold be easier to shorten the plug threads.

[rich pierce]

It’s enough for me.  If you can find a nut that threads onto the plug, that helps square it up after cutting some off. Spin it on before cutting obviously.

[45-110]

just me, but i would not be comfortable with .394 of thread. seems to be a trend of late to offer the minimum thread length which has been discussed here ad nauseum. 1/2"- 9/16" would be what i would like for a good margin of safety, for you or the next owner.
kw

[D. Taylor Sapergia]

The plug threads in my 15 bore Staudenhymer (sp?) double barrels are .375" long.  They still have case colours present and there is no shoulder for them to bottom against.  Your choice, but if it were mine, and the threads were tight, I'd be fine with it too.

Great tip on squarely cutting off a thread journal, Rich!

[wapiti22]

At .394 depth, I'll have about 6 threads engaged.

[Craig Wilcox]

Historically, 3/8" was used.  We modern types seem to prefer 1/2".

[Leatherbark]

If you decide to lengthen it by hand you can buy you an end mill bit of the same size as the drill for a 5/8 x 18 ( I assume its that size) and deepen it before finishing up with a bottoming tap.  You can get the endmill bit off of ebay.  It will have a 1/2 inch shank and you should be able to drill it slightly deeper and tap the hole deeper.  This will leave you a nice shoulder for the plug to seat against.  If you're going to get into this hobby you'll use the endmill and tap more.  You should deepen the hole slightly with the correct size tap drill and finish with the end mill.  Keeping the barrel square while doing this will be vital.  That  barrel will drill like butter with a big .45 pilot hole.

Bob

[45-110]

the correct way, and cheapest is to bore the tap drill diameter with a boring bar, not buy a special grind diameter end mill. its a simple lathe set up for sure, but guarantees positive results.

[Bob Roller]

the correct way, and cheapest is to bore the tap drill diameter with a boring bar, not buy a special grind diameter end mill. its a simple lathe set up for sure, but guarantees positive results.

45-110 is right. A lathe and a boring bar works well.Custom ground end mills are OK but
for a ONE TIME job not very practical unless you have a big budget for hobbies and such.
I have used the boring bar at Bill Large's shop more than once to form a flat shoulder
for the plug to come up on and seal properly. Never had one leak that I know of.

Bob Roller

[bptactical]

I sent an offer to the OP to either push the existing threads forward a bit or lop it off and start fresh.

[T*O*F]

It's perfectly safe as is.  However, before making any decision, locate where your touch hole will be with each depth.  Then consider how that will affect lock placement, which in turn will affect your side panels and perhaps your wrist.  Then make your choice.

[Dphariss]

If you have a lathe this is the best way. Center the bore at both ends of the headstock and then bore it. Leaves a square shoulder, may need two or even three different taps to get it all the way to the bottom. This barrel is 4150 BTW.
Large bore, thin wall barrels like shotguns/fowlers there is often no shoulder. But the taps and dies used would cut threads on the plug that were very close the the hole the tap would make. Modern taps and dies have "tolerance" so its best to order taps with the minimum clearance most are typically H3
But H1 taps will give a tighter fit and better engagement.
http://www.tapmatic.com/tapping_questions_class_of_threads_h_limits.ydev
Dan

[Dphariss]

The problem with end mills is that then are expensive in odd sizes and they cut on the sides. Its possible to make a cutter to even, reshape or deepen the hole by using a piece of rod that will just fit in the tapped hole, drill a hole in the end then file teeth and harden or case harden. This will cut slow but will work if the teeth are cut right and its a lot cheaper. I have one I drilled and tapped for a hex head bolt then use a 3/8" drive ratchet to drive it. An electric drill will work too. Just drill and tap the none working end then screw in a bolt that the drill chuck will accept, cut the hex head off. But be careful with the power tool than you don't damage the barrel threads or overheat the cutter. The bolt in the end can also be used to spin it against a belt sander to reduce the size to fit the existing hole.
A sharp chisel can be used to upset teeth I suppose. But I filed mile with a 3 square file with one flat ground smooth the form sharp edged, as used for dovetails.

Dan