doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Pressure question

A simple question, I think. I recently bought a British gun and I plan on loading ammo for it. The gun is marked three tons, so I know I want to keep my ammo below 8900 PSI. I'm thinking I'll stay below 7000.

My question is this. What is the pressure level in Lup that I want to stay below?

I've tried searching for the answer and all I could come up with were complicated discussions without a clear answer.


Thanks, Tim

Congrats on the new to you gun.

I’m pretty sure the conversion of LUP does not correspond exactly to PSI. Are you getting reloading data that expresses pressure in LUP?

7000PSI is pretty low on the ammunition pressure scale. Bet you are fine with any ammunition around that point.

Good luck.

Best,
Ted

Tim, good question, but not a simple question at all. There is no direct conversion formula to convert psi to LUP, or vice versa. Supposedly, some of the so-called conversion formulas can be far enough off to provide dangerous and unreliable information. The best you can get is approximations that may or may not come close to being accurate.

I've gone down this rabbit hole before, and found much of the same goblety-gook that you are probably finding from internet shotgun experts.

Fortunately, it probably doesn't matter much because virtually all shotgun pressure measurements being taken today rely upon piezo-electric transducers, which react much differently than directly measuring the compression of lead or copper crushers.

Probably more than you want to know Tim.
In the 1954 British Rules of Proof the service pressure was expressed in TONS and was measured by LUP.

There is no mathematical conversion for LUP as measured by change in dimensions of a lead cylinder in a crusher apparatus and a Tarage Table, to PSI as measured by modern piezo transducers.
Adding 10 - 14% to LUP numbers works pretty well.

The 1954 Rules of Proof “Highest Mean Service Pressure” equivalent transducer values as converted from LUP by Burrard’s formula: estimated Long Tons/ Sq. Inch Lead Crusher Pressure (Cp) conversion to PSI (pound-force per square inch): (Cp x 1.5) - .5 = TSI, TSI X 2240 = PSI.
2 3/4 tons = 8,120 psi = 560 BAR
3 tons = 8,960 psi = 618 BAR
3 1/4 tons = 9,800 psi = 676 BAR
3 1/2 tons = 10,640 psi = 734 BAR
4 tons = 12,320 psi = 850 BAR

Great Britain joined the Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives in 1980 but the new Rules of Proof were not introduced until 1984. Lead Crushers were used however until 1989; the proof marks in BAR (and shell pressures) thereafter are PSI by transducers.

I don’t know what powder you are using but there a number of loads listed within your criteria utilizing Hodgdens Perfect Pattern. In my neck of the woods it seem readily available.

The plan is card and fiber wads, I have PB and 7625, and the load data is in LUP. I'll just keep pressures for that down around 7,000 too, I would think that should be safe.

Tim

Are you going to tell us about the gun?

Best,
Ted

Well, since you asked here it is. Joseph Bourne and Sons, 28-in barrels, 6 lb 11 oz, claimed cylinder and full, ejectors. Gun is in somewhat high condition , I suspect it's been in for regular maintenance. Action is absolutely tight and on face.

I tried, I sincerely tried posting photos. It's just impossible. If you know how to get the photos off Flickr, explain it to me and I will post them.

Tim

Tim,
Just email them to me, and I’ll get them up on the board for you. Just as an aside, I have to take them off of flicker to get them to work anyway. At the moment, the Brits that post here, and put up with us, can see the pictures with the service I use. That seems to be valuable when posting photos of English guns.

Perhaps one of them will have some better insight on your gun?

tedjs@usfamily.net.

Best,
Ted

____________________________________________________________________
Helps me to keep my curmudgeonly image here cemented.

I have bid on a few Bourne guns, but have not bought any yet. All pretty nice, from what I could tell in the photos.

I emailed you the pictures, at least I think I did, I don't know how the hell anything works on this damn tablet. If they didn't come through, or something is wrong let me know and I'll try again.

Tim

Haven’t seen them, Tim. Try just sending an email, no attachments, and I’ll send one back.

Should clear the spam filter if you do that.

Best,
Ted

BOOM!

Here they are:







Good looking gun, Tim.

Best,
Ted

______________________________________________________________
Church windows!

Very nice Tim.

I'd be happy to own that.

I am curious why it was reproofed in both 1985 and 1986, but it passed both times, so it doesn't really matter.

Tim: the pic Ted attached is very small and I couldn't enhance it adequately to read the proof marks and date codes.
If you would like, send me a full size, high resolution close up as a jpg attachment, not buried in the email, to drewhausemd@yahoo.com

When in doubt send a load out to have it tested for pressure. Best money you will spend this year. I have a standard load based on Fiocchi hulls trimmed to 67Mm. Pressure and velocity are known and I have them in shot sizes from 7 to 9's. I trimmed 5,000 hulls and loaded them all over a weeks free time. They sit boxed on the shelves and are my basic go to load for all my 12 double needs except non-toxic needs. The only thing I load in Fiocchi hulls is this single load so just be sight I know that the load is safe for all low pressure uses.

Drew, the reproof marks are MC and LC:

if you have supplies of both PB and 7625, this reference will offer you numerous loads - down to pressures that are amazingly low. my one comment regarding the gun is that there are older bore proofs 13/1 (right tube), and a faint 12 (?) on the left tube. that could be a tube originally as tight as .719", and last time through the process measured .736" - and that's the exact scenario that makes me keenly interested in knowing wall thickness. i have several guns in this same general condition - nicely cleaned up bores - but marginal wall thickness. i think it smart to take that in consideration with load pressures. http://pdf.textfiles.com/manuals/FIREARMS/imr_reloading.pdf

very handsome old gun, use it in good health.

How'd you do that Geoff!?! Very interesting, and I've never seen that.

I downloaded the photo, zoomed in, then used the Windows Snipping Tool to clip the relevant part of the image.

I like to think that 36 years in print and graphics wasn't a complete waste of time.

Not as good as Geoff's image, but better



Original Birmingham Proof 1925-1954 at 3 Tons = max. service load about 9000 psi
13/1 bore .719 - .728" and at reproof as greybeard said .736"

Can't explain 2 Birmingham reproof date codes, but some alteration in the barrel had to have been made.

I don't know what the M over 928 means?

I can't find that anywhere either.

So, I had the gun up to my gunsmiths to look at the barrels. He has a bore gauge but it's just an inside gauge, when he measured he came up with a measurement of .734, the proof house came up with .736. We measured the outside barrel diameter at.800, so subtracting 736 and dividing by 2 gives me about 30 thousandths wall thickness.

I was concerned a little about the wall thickness when I bought it, but it seems okay. You gentlemen know a lot more about this than me, I'm just guessing that since the gun was set up to weigh 6 and 3/4 lb instead of 6 and 1/2, and the barrels are only 28 inches instead of 30, they struck the barrels a little heavier to get the weight up.

Tim

others will show up here with more nuanced knowledge - but my general comment is that the difference between average outside diameter and average inside diameter is not a reliable means of determining minimum wall thickness. i have struggled with this exact situation, i bought a manson wall thickness gauge to compliment a couple of interior bore gauges, inside micrometers long enough to measure most chambers and chokes, and crummy choke gauges from earlier days. i discovered that the wall gauge is a great challenge for me to use. i was paralyzed briefly as a 5 y.o., and have some lifelong damage that manifests itself as a fine motor tremor or shake. i'll soon turn 77, and age has certainly not brought any improvements in that regard. in spite of many helpful tutorials here, regarding best practices to achieve consistent results - i simply cannot handle the leveraged micrometer smoothly enough to trust. most of my guns were measured with assistance from several "steadier" friends, and i abide with the generally stated opinions that below 25 thou, you should pay attention - and below 20 thou - a lot of attention.

You have my admiration for not being a quitter, graybeardtmm3. Well done, very well done.

When we measured the barrel we got consistent ID measurements of .734 rotating the gauge, the OD measurements varied by a few thousandths but was not less than .800, so I figured I was on pretty safe ground.

How much of a variation from my measurements do you think there would be if a dedicated wall gauge was used? As far as ammo goes, I don't see any need to break 1200 FPS, and I shouldn't have any trouble keeping pressures below 7000 PSI.

I'm sorry to hear about your tremors, I have some issues from a past operation, and I will never be a good offhand rifle or pistol shot, so I have a small idea of what you are going through. I hope your problem doesn't get any worse.

Tim

Bores may not be centered in the barrel. Your assumption that the .066, .800-.734=.066, is evenly divided in all directions, is not certain. So your .066 may be .033 of both sides and could be .044 in one direction and .022 in another. Worse when you have a gun which has had a dent removed and the barrels filed down and refinished you will have a thin area. Not trying to confuse you or alarm you. But we measure wall thickness not internal and external dimensions and do a little math. And measuring is as much an art as a skill. Good equipment, poorly used, will not give you true results.

Tim, the only way to know barrel wall thickness for certain is by directly measuring all areas of the barrels. With SxS double barrel tubes, this means that no shotgun barrel wall thickness tool can measure the areas between the ribs. On the other hand, the area between the ribs is more likely, but not guaranteed to be thicker than an area at the same point from breech or muzzles that is situated on the outer left or right sides. The external areas away from the ribs are finish struck and polished at the factory, and they may have additional striking and polishing done afterward if the barrels are repaired or refinished.

If you look at photos of SxS shotgun barrels that have suffered a burst other than a chamber burst from an overload, etc. the damage is typically a split or bulge accompanied by splitting, and it virtually never is located right along the ribs, because wall thickness is generally a bit thicker there. Even when the cause is an obstruction like mud or snow, the failure is still most likely to initiate at the weakest point radially.

The reason for this is because once the tubes are joined, no further external striking of the tubes can be done between the ribs. Also, the area between the ribs is not subject to denting, so there is no chance that repairs that involved striking would create thin spots. Internal polishing or honing is likely to be fairly centered in the bores, but there is also no guarantee that the operator of the hone didn't exert force that was uncentered. So there are just too many variables to simply subtract I.D. from O.D. and think you have an accurate wall thickness. Even people who buy barrel wall thickness measuring tools seem to have a lot of problems getting consistent and accurate readings. I'd guess the vast majority of shotgun owners have no idea what the barrel wall thickness of their guns actually is. For the most part, ignorance is bliss. But if you have doubts, it would be prudent to have them checked by a competent gunsmith.

After digesting all the responses and doing a little research into thickness gauges, their accuracy, and Dewey Vicknaires opinion of them, all of which is something I don't know enough about to debate, I suppose I just have one question left. Going off the marks on the barrel flats, is this gun still in proof that 3 tons? I suppose that is all that really matters.

Tim

The gun will be in proof until the bores are .010 over proof. With metric measurements I think it is .008 now. Any way, yes your gun is in proof. We put too much stock into believing proof means it is perfectly safe. I put a lot more stock into wall thickness. I know of two0 guns which recently passed re-proof with barrels as thin as .015 and .016". Now consider they will still be in proof with .009" removed from the bore. That brings them down to .006 which is the thickness of a Coke can. In proof is not everything. .020, in 12 bore gets my slow down attention. That is why I bought wall thickness measuring tools and learned how to use them. If you go to a major SxS shoot see if anyone there can measure the wall thickness for you.

See the 3rd post. The gun was proved in 85' and again in 86' when the Proof House was still using crushers. It was proved then under the 54' - 89' Rules at 900 Kg/cm2 for a service pressure of about 9500 psi by transducers.
Starting in 1989 it would have been proved per CIP at 850 BAR for a max. service of about 10,700 psi by transducers.

It was in 1986. That may seem like a non-answer, but we don't know if someone messed with the gun in the intervening 40 years.

The gun looks like it's in good condition, which tends to make me think it's been well maintained...but no one can say with 100% confidence that someone didn't mess with it and it was cleaned up and refurbished after that without re-proofing.

If I wanted to be certain, I'd have someone I trusted with a wall thickness gauge go over the gun carefully if I were worried about potential thin spots.

I'm running my wall thickness gauge over all the guns I'm buying, and getting better at it, but if I saw a spot that concerned me, I'd have it checked.