Slugging Trapdoor Barrel

[IrishJoe3]

Gents,

I was hoping to get the opinion of more experienced bullet casters and shooters of the US Trapdoor Springfield 45-70.

I have been shooting my trapdoor using the Lee .459 hollow base 405 grain bullet (1:20 ratio lead to tin). With that bullet I get mediocre accuracy (18" at 100 yards). 500 grain bullets swaged to .459 tumble. I've long suspected the bore was oversized (common in the trapdoors).

I'd like to purchase a custom mold to match the actual bore diameter for this rifle.

I took an oversized soft lead slug which I rolled to a diameter of .463. I coated the bullet in layout fluid (blue). I then hammered the slug through the bore.

Now the Trapdoor rifling is three lands and grooves of equal width, making it impossible to get groove width off a slug with standard calipers. Measuring as best I can with my calipers I'm getting about .462. But again that's across lands and grooves and not a true measurement. Looking at the slug you can see the wear in the layout fluid was not scraped off in the grooves and the surface of the slug in the grooves looks like it had a light contact with the barrel wall.






The question is what would be the best diameter for this mold or what should my next step be to find out?

[870classic]

IrishJoe,
You will probably need to use a "3-flute tri-micrometer" to obtain a close measurement for the bore.

Below is a good reference for your review. The author also has a Trapdoor Springfield.

http://www.texas-mac.com/Measuring_Groove_Diameters_of_Odd_Groove_Bores.html

Best regards,
870classic.

[kendog4570]

There is some trig involved to get the right measurement. Damned if I know how to do it.

Your bore is not over sized. That is a common misnomer about the Trapdoor. It is exactly the dimension they meant to cut it when they made it. It was designed to shoot dead soft 500 grain bullets sized approx .459 that slug up to groove diameter upon firing. The original charge was compressed into the case and made the old gal bark, not foof. The solid 405 grn. carbine bullet wouldn't obturate with the reduced weight and reduced powder charge so it was redesigned with a hollow base to do the same. This process allowed as many as 20 or 30 shots before the bore needed wiping.
Get Pat Wolfs book on loading for the trapdoor.

https://the4570book.com/

[kendog4570]

Saeco makes a mould copying the original M1881 bullet.

https://www.buffaloarms.com/460-500-grn-rn-government-saeco-bullet-mould-1-cav-1881-sae61881

[IrishJoe3]

Saeco makes a mould copying the original M1881 bullet.


https://www.buffaloarms.com/460-500-...-1881-sae61881

I have tried that bullet and unfortunately it tumbled. I do have Wolf's book. An excellent resource and I have followed his instructions to the T but could never get more than mediocre accuracy. The bore is in decent shape, and I've tried that Lee hollow base with black powder, smokeless, resized, as cast, never had much luck.

[kendog4570]

I have tried that bullet and unfortunately it tumbled. I do have Wolf's book. An excellent resource and I have followed his instructions to the T but could never get more than mediocre accuracy. The bore is in decent shape, and I've tried that Lee hollow base with black powder, smokeless, resized, as cast, never had much luck.

Hmmm... I had good results with it with Wolf's method. Didnt put it on paper but could hit bowling ball size rocks at 3 and 400 yards easily. Both in a Trapdoor and Shiloh Sharps '74. Very consistent, way better than more modern bullet designs without the compressed powder and primer/flash hole mods etc.
Possibly something else at issue with your particular rifle?

[smle-man]

Are you sure the crown/muzzle isn't worn oblong from cleaning?

[kendog4570]

Check for barrel bulge(s) and headspace. A bulge will destroy accuracy.

[870classic]

There is some trig involved to get the right measurement. Damned if I know how to do it.

FYI, the trigonometry math is covered in the hyperlink provided.

Best regards,
870classic

[IrishJoe3]

Well, I sent a diagram of the original M1873 bullet to Accurate Molds and he created a pretty close mold and added to to his catalog. It is not a hollow base. And he can make it in any diameter up to .469

$77 is worth trying out.

Diagram from Pat Wolf's Book


Mold created by Accurate Molds

http://www.accuratemolds.com/bullet_detail.php?bullet=46-412G

[IrishJoe3]

Check for barrel bulge(s) and headspace. A bulge will destroy accuracy.

I don't believe the barrel is bulged, when I slug it there is fairly consistent resistance all the way down; I don't feel any loose spots.

Are you sure the crown/muzzle isn't worn oblong from cleaning?

Good call, Inside the muzzle it is worn from cleaning rods which could be part of the problem. The muzzle still measures round, but the rifling in the last 1/4" or so is worn flat

[highpower]

I know people hate the idea of it, but there is a reason that government's would counterbore rifle barrels to get more life out of them. I think that you should look into having it done on your Trapdoor, you really have nothing to lose.

I used to be able to hit a 24"x24" gong at 400 yards with my 1884 Trapdoor rifle with a reasonable degree of regularity back before my eyes took a dump. It was literally like lobbing mortar rounds at it.

Unfortunately, I don't remember the bullet diameter or load I was using to do it.

[smle-man]

I don't believe the barrel is bulged, when I slug it there is fairly consistent resistance all the way down; I don't feel any loose spots.




Good call, Inside the muzzle it is worn from cleaning rods which could be part of the problem. The muzzle still measures round, but the rifling in the last 1/4" or so is worn flat

The muzzle is your problem. I've seen rifles with inches of smoothbore throat and pitted rifling that shoot fine because the muzzle is perfect. I had an FN commercial rifle if a perfect bore that keyholed .30 bullets because the muzzle was screwed up.

As was suggested, counterbore the muzzle and see how it shoots.

[Paul_R]

.462 is pretty standard for trapdoors, believe it or not. Soft lead hollow base bullets work well if you have to shoot undersized bullets.

MP makes the best bullet mold for trapdoors I ever found:

https://www.mp-molds.com/product/mp-462-420-hb/