Trapdoor ........... perhaps

[Tomklinger]

Gentlemen,
The hole for the pin in the block is not round, it’s oblong by .005 to .007 longer than it is high. The block is designed to have a few thousands forward to rear movement. When the block is closed and latched, it is moved forward against the pin. There is several thousands clearance in FRONT of the pin. That way there is no pressure on the pin when the rifle is fired. The force is all directed to the rear of the block and tang with no pressure on the pin. Hence a smaller diameter pin allows the block to lock up forward towards the base of the ctg. That oblong hole is why there is always so much looseness felt in the block when the block is open.
Tom

[mdeland]

Thanks for the conversation guys it is both enlightening and interesting. It looks to me like the lock cam is designed to stop only the vertical movement of the block against the angular block seat at the back of the action.
The front of the block is held down by the block projections that cam into the under side the cross pin ears.
Once the block is locked down it looks like the pin could be removed and the gun could be fired in perfect safety. Dehaas tells of the same with the Martini actions. None of the firing pressure load is ever on the cross pin on either action ,they are just a hinge that holds the action together.
As I look at the back of the block I can see the smoothed out, impact seat marks that are taking up the back thrust against the angled back of the receiver.
Also the cam is quite loose in my gun and is held in by a side plate and one screw which clearly is not designed to hold any back thrust pressure.

[Luke]

The Trapdoor is known as a "yielding breech" design. Neither the hinge pin nor the cam shaft take any load. Rearward thrust upon firing moves the block back a few thousandths, bringing the curved inner surface of the cam recess inside the block in contact with the forward surface of the cam. This in turn brings the rear cam surface in hard contact with the curved surface machined in the tang screw. The geometry of the cam in combination with this surface then transmits this force downward, seating the block tighter into the trough. The ears underlapping the front hinge prevent the front of the block rotating upward. There may be minimal contact of the rear block face, but this is incidental, with the intended load path thru the cam.

It is the fit of the cam that determines headspace upon firing.

[mdeland]

I have an older good friend who is quite knowledgeable about Trapdoor guns and once did a blow up test on I think an 84. He told me they kept increasing the charge they put in the gun to what would be dangerous in any make or model and they could not get it to blow up or even malfunction. Finally they put in such a huge blue pill over load that the action let go. Don said they waited for few seconds to make sure it was safe before approaching to see the destruction and after picking the wrecked action up and looking it over the breech block came down and made a thud in the dirt beside them! That block must have gone half way to the moon!
He said he had always heard that trapdoor actions were weak but after that deal he had new respect for the strength of the design.

[mdeland]

I think I see what you guys are telling me. I'll take the cam out of the block and look it over closer inside. Fun to learn new stuff, thanks !

[mdeland]

Guys, if you happen to have a copy of Frank DeHaas Single Shot Rifles and Actions , on page 91 there is a cross section of a 73 Springfield Trapdoor that shows the position of the cam in lock down. I think you will see the reason I am skeptical that the back thrust on the block is arrested by the cam rather than the block itself. In the drawing the cam can only be arresting the vertical movement of the block.
You will note the block making physical contact with the top and bottom of the rear of the receiver and potentially the threaded tang face.
Note the cam pin is below the contact with the back of the tang mortise.

[mdeland]

Well I went out and took out the cam and sure enough you guys have it right! The back thrust is clearly through the cam after all. The cam in my gun shows impact shine back of the block, onto and through the cam to the tang mortise seat. The cam in my gun turns in more than is shown it the DeHaas cut away and is a straight block impediment against the back of the tang mortise just as you stated.

[mdeland]

I just went out in the shop and slugged the bore, run it through the Powley gauge and the groove diameter came out to .4621 so the grooves are a bit over .006 deep. Wow that makes my .457 Lyman .005 to narrow. Wonder if she will lead up? Perhaps the cork wad will help out.
Looks like I'm going to have to cut another bullet cavity for this bugger any way.

[alfajim]

I just went out in the shop and slugged the bore, run it through the Powley gauge and the groove diameter came out to .4621 so the grooves are a bit over .006 deep. Wow that makes my .457 Lyman .005 to narrow. Wonder if she will lead up? Perhaps the cork wad will help out.
Looks like I'm going to have to cut another bullet cavity for this bugger any way.

You should pick up a copy of Spence Wolf's book Loading Cartridges for the Original 45-70 Spr4ingfield Rifle and Carbine. Everything you would want to know about shooting a trapdoor is in there loads, ballistics, velocity's bullets, specs etc. 90 percent of all the data is from the original armory sheets and books plus dissecting original cartridges. It;s available from BACO and the correct dies and stuff from Track of the Wolf.

Also a copy of The 45-70 Springfield by Joe Poyer & Craig Riesch it has all the history and specs on all the different models. Last but not least is More Springfields 1873-1893 by Richard Hosmer and there is Al Frasca at Trapdoor Collector he has parts and books very knowledgeable Guy on Trapdoors.

Jim O

[77 sharps]

mdeland
I would think that a felt wad would cushion the bullet base reducing the expansion of the bullet making leading more likely. A simple card wad or a lube wad would work best. I shoot PP bullets 0.004 under bore diameter for hunting with reasonable accuracy and no burned patches.

[Luke]

The grooves of the TD barrel are deep,(.005+/-.001) and the lands consist of 50% of bore circumference. This is different from modern barrel thought, which has land circumference % around 10 or so. The Army loaded undersized bullets to allow the lead displaced by these lands to flow into the grooves while not creating 'finning", or rearward displacement around the edges. This depth also allowed "windage" for fouling when fireing extended strings of shots, ie a firefight. I do not believe the current thought of "bore diameter or .001 over" is optimum for the 3 groove TD barrel. The Army started off with a .455 bullet, then moved up to .459, but no larger. I shoot .459s in my TDs, and have been pleased with their performance.

[Luke]

Meant to say groove diameter.

[mdeland]

Thanks for the heads up and shared knowledge guys! I got to thinking about the .457 bullets I have loaded up and had the same thought that the broad lands may very well displace lead enough to fill the equally wide grooves. It will be interesting to see if they will work in the .005 wider groove diameter.
Good point on the wads, I usually use veggy wads .062 thick I make out of head gasket material. I just happen to have some old cork wads I wanted to use up.

[mdeland]

I'm pretty sure I'm going to lap this barrel a bit and when I do I'll post some pictures of it! I couldn't feel any tight spots in the bore with the plug gauges and that usually means the grooves follow the same contour consistency as the bore is the bed for the rifling head to cut the grooves. A pitch change or a hard spot that deflects the cutter can alter things enough to feel like a tight spot but this is in the groove not the bore. A fresh lap slug will pick this up but there isn't much I know of to do about that except to lap the transition as unobtrusive as possible.
Actually the first couple of trips up bore with a fresh lap slug that is not charged yet will tell a lot about what shape things are in.

[Michael Johnson]

Mike, I have two Springfield Trapdoor rifles. An 1878 and an 1884. Steve Brooks made me a mould that replicates the 1881 Armory bullet. It weighs 500 grains cast in 20/1 alloy and measures 0.460. It shoots well out of both rifles. You are correct, their grooves are deeper.