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

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.