Patent breech in underhammer

[Metalshaper]

Randy,

I use a lot of 1/4 X 28 threaded nipples.. and was always told my brother in law, who is a mech Engineer.. it only takes three complete threads to create a full strength mechanical hold.

if you consider the information here http://www.gizmology.net/nutsbolts.htm

this page mentions anything over 6 threads is basically superfluous.. calculating that each full thread of a 1/4 X 28 is .o35714" approx...
6 X 0.035714 = .21484

So if your barrel has a .217 wall?


never hurts to go ahead and use a chambered breach.. they can be made in a home shop, as I described in a previous message..
but it never, ever, hurts to have it professionally made?

Respect Always
Metalshaper/Jonathan

 

[paulvallandigham]

It depends on how fine the threads are on the nipple you choose. Some of the metric thread sizes are smaller than US Fine sizes. Call Track of the Wolf or Pecatonica River and talk to them about this barrel thickness issue. I am sure they can give you sound advice. My math is working differently from yours. But, I went back and checked your prior posts here, and you don't mention if you are using a .69 RIFLE barrel, or a .69 cal. Smoothbore barrel. If a smoothbore, then subtracting .69 from 1.25 = 0.8625" Divided by 2 = .43125" thickness to the wall. That would be more than enough metal to screw in a nipple.

However, if this is a rifled barrel, we need to know the Groove diameter, to calculate wall thickness. I had a Spanish made percussion rifle, with a nipple and drum arrangement. The nipple screwed into the drum. I don't believe I had as much metal as your .217" measurement gives you, in the wall of the drum.

If you can get three complete threads in the wall thickness, you should be good to go. Choose the fine thread for the nipple accordingly. For instance, a common 1/4-28 nipple size has 28 threads per inch. That is 0.0357" per thread, or more than 6 treads in a steel barrel wall that is .217" thick. You can also buy a 1/4-32 tpi nipple, giving you almost 7( 6.9) threads in your .217" wall.

Can I ask why you want a .69 cal. rifle barrel? My gunmaker has a .62 cal. rifle that he built for himself, and it will take just about all the big game that walks or crawls on this continent. The .62 cal. lead ball weighs in the 320-325 grain range, and is a HUGE freight train let loose on any animal.

With that thin a wall, I would be more worried about cutting dovetail slots for barrel hangers, and sights, than with the nipple you use. Just my humble opinion, and worth exactly what you pay for it! Best wishes. :thumbsup:

 

[ripshod]

In reply to paulvallandigham about a .69 cal rifle my answer is I don/t have one.I really don/t have a need for anything that big.With the underhammer it is actually not to expensive to assemble barrels in different calibers.

 

[bob308]

i make my own also make my own false muzzles. yes i have a lathe and a mill. no by guess work.

 

[Zonie]

randy p
I think Pauls minimum thread length for maximum strength is close to right with his 3 thread rule.

Using a 1/4-28 thread Pauls 3 thread rule indicates that a length of .107 full threads is adequate.

It may be interesting to point out here that the number of full threads on 4 different rifle nipples I have with different thread sizes are, 5, 5, 4, and 3. All of these are fully capable of working in a .32 caliber gun without blowing out if the threads in the breech or drum are in good condition.

I mention the .32 calibers upper pressure range (Lyman found some charges gave pressures of over 25,500 PSI) because the small bore guns develop higher pressures than the large bore guns.

A .69 caliber rifle will have lower pressures than a .54 caliber gun has and the maximum pressures Lyman lists in my BLACK POWDER HANDBOOK for a .54 cal is a load shooting a 450 grain slug over 120 grains of Pyrodex RS which developed 24,500 PSI (pg 253). (I should mention that the same bullet/barrel using 120 grains of GOEX 2Fg developed a pressure of 16,900 PSI so if the synthetic black powders are avoided the pressures are much safer).

Another bit of trivia: At a breech pressure of 25,500 PSI, the maximum actual force a 1/4-28 threaded nipple must withstand is 1252 pounds.
A standard steel bolt screwed into a standard steel nut with 10 Ft/Lbs of torque will create a clamping force of 2,400 pounds so you see, the nipples threads are quite safe even when unrealistically high breech pressures exist.

What all of this boils down to is a standard nipple will be very safe in your proposed gun.

Speaking of the breech plug threads:

The breech plugs minor diameter must be larger than the maximum bore so using a .69 caliber with .015 deep rifling we have an effective diameter of .720.

The minor diameter of a 3/4-16 female thread is .696 max..
As this is smaller than the proposed rifling, it will not work. You must use the next larger standard size breech thread which is 7/8-14.
The minor diameter for a female 7/8-14 thread is .814 max so this should work.

Personally, I would not use the 3 or even a 4 thread minimum here. It's one thing to have a nipple blow out but having a breech plug blow out could be catastrophic.
I would use the industrial standard of 85%D which is commonly used to determine the thickness of high strength steel nuts.
The result would be .85 X .875 = .744 min breech plug thread length.

 

[Richard Brinkman]

There's some usefull info right there. I'm surprized that the 32 had such high pressures, assuming that was with a round ball. Wonder if anybody has any information on pressures for the long range shooters? Say a 550 grain paper patch bullet with a 80 to 100 grain charge of Goex 2f. I've looked around the long range sites and not really found much along these lines other that most use Swiss which burns with more pressure that Goex.

 

[Metalshaper]

RH,

Look into it and see about the long range guys using platinum lined percussion cones!! the heat and pressures of their loads, can burn out a normal cone in just one cycle of shooting.

See if Art, aka Snapper, jumps in on this one. Him and the guys he runs with shoot the Long Range matches and such..Think he even shot with the US long range team, this last go round?? if I'm not mistaken?

Respect Always
Metalshaper/Jonathan

 

[Zonie]

rhbrink:

You didn't mention a caliber but the Lyman BLACK POWDER HANDBOOK & LOADING MANUAL (2nd Ed) shows a .45 caliber, 28 inch, 1:48 twist barrel shooting a 325 grain slug over GOEX 2Fg powder. The pressures shown are:
80 grains = 16,000 psi, 90 grains = 18,800 psi, 100 grains = 21,600 psi, 110 grains = 23,400 psi, 120 grains = 25,200 psi.

It also lists a .50 caliber 22 inch 1:24 twist barrel shooting a 490 grain slug over GOEX 2Fg powder. The pressures shown are:
80 grains = 17,000 psi, 90 grains = 18,000 psi, 100 grains = 19,000 psi, 110 grains = 20,200 psi, 120 grains = 21,400 psi.

As I mentioned earlier, the smaller caliber guns develop higher pressures. Notice that even with the heavier slug the .50 caliber pressures are lower than the .45 caliber.

Folks should also notice that although Lyman tested some of the synthetic powders and their pressures were extremely high (like the .50 mentioned above developed an awesome 29,000 psi using 150 grains equivalent of Pyrodex pellets they DID NOT USE 3Fg POWDER IN ANY OF THESE HEAVY SLUG TESTS.
IMO, 3Fg powder should not be used with ANY slugs. It burns way to fast and produces too much pressure.

 

[TARL01]

Zonie, how do these higher pressures translate into feet per second travelled? I would assume that the smaller calibers will travel faster, but will hit with less footpounds of force. Is this the case?
Woody

 

[roundball]

Actually, 3F can be used for anything...I use it all the way up to .62cal for both the large heavy PRBs and large heavy shot charges.
Its all the same blackpowder...made and screened out of the same manufacturing run...at the same time 2F and 4F are being screened out.

The size of the 3F powder charge being used simply needs to selected / reduced appropriately to keep pressures in the same ballpark as if using a desired charge of 2F...the standard rule of thumb being a 10-15% reduction when substituting 3F for 2F. IE: an 85grn charge of Goex 3F instead of 100grns of Goex 2F.

 

[Zonie]

We are not talking about patched roundballs here.

We are talking about heavy lead slugs and they will produce VERY HIGH PRESSURES even when 2Fg powder is used.

I will repeat myself:

DO NOT USE 3Fg BLACK POWDER WHEN SHOOTING HEAVY LEAD CONICAL BULLETS IN YOUR MUZZLELOADER.

When shooting patched roundballs some simple thing like "reduce the 3Fg powder load 15 percent" works all right but when dealing with the pressures created by heavy slugs it is downright dangerous to use such a guideline.

Tarl: The smaller caliber was shooting at a higher velocity but I think this is due to the lighter bullet weight.

 

[ripshod]

I have been shooting 75 grains of 3F Swiss behind a 460 No Excuse bullet in my Green Mt. 1 in 28 twist Renegade flint barrel.Very mild shooting and extremely accurate.I am surprised to find that this is a possibly unsafe load.

 

[Richard Brinkman]

Just curious if that is a drop in barrel it probably has some type of chambered breech? If so are you using a flashhole liner and have you noticed the flash hole being burnt out? Just asking try to learn as much as possible about this heavy bullet stuff.

 

[ripshod]

The touch hole liner may be wore out,but I can not blame it on the load as I have moved my liners around between guns and I am not sure how many shots are on it