Bore size vs bullet size

[hydraulic]

Since it is difficult to measure a slug from a three groove Springfield trapdoor bore, I drove a soft lead 500 grain Lyman bullet down the bore in an effort to get some idea of the size of the bore. It measures .452-.454. I have been shooting this same bullet in WW's as they come from the mould, and they
measure .455-.465, the higher reading coming from the seam. I took the plug out of a Lyman .459 sizing die, and the soft lead slug would drop right through. Obviously, the bore on that old trapdoor is a lot smaller than I thought. The question is-can a bullet that is too big for the bore result in erratic accuracy? One day I can shoot 2 1/4 inch 5 shoot groups at a hundred yards, and the next day they scatter out to 5 to 7 inchs. I pushed a half dozen bullets through that .459 sizer with my thumb so I'm going to load those and see what happens.

[Don McDowell]

Bullets that are to big, say over .003 over groove, will "fin". The excess lead displaced from the lands will be shoved over the base in a fin like fashion, so you end up with a ragged base , and that won't do much for accuracy.
Might try sizing all the bullets to .459 or 8 and see if that helps get things a bit more consistant.

[NickSS]

I do not understand your measurements from your slug That you got. Trapdoors Springfields were supposed to have .451 to .453 bore diameters and the groove diameters were supposed to be .461 to .463. Of course some were larger but I never heard of one smaller. Was your rifle re barreled?

[martinibelgian]

Well, it is 3-groove rifling - so with a normal caliper, you will be measuring land to groove - not good! You cannot accurately measure odd-grooved rifling with a caliper, but you can get an approximation by rotating the bullet in it with slack jaws, taking the highest reading. The same error happens with people when slugging their martini's - odd-grooved rifling.

[montana_charlie]

I, too, am having a little trouble understanding what you learned with your measurements.

I drove a soft lead 500 grain Lyman bullet down the bore in an effort to get some idea of the size of the bore. It measures .452-.454.
Is this the 'bore' measurement, or 'groove' diameter? My first assumption is to take it as 'groove', but your words seem to imply you were looking for a 'bore' measurement.
I'm not real sure which part of the slug you gave us numbers for.

If .452 is 'bore', and .454 is 'groove', your rifling is only one thousandth deep.
That's rather hard to accept without your verification.

If .452-.454 is a range of dimensions that your 'bore' diameter falls within, it still leaves us wondering what 'groove' is.

I have been shooting this same bullet in WW's as they come from the mould, and they measure .455-.465, the higher reading coming from the seam.
I can't say I've ever heard of a bullet that far out of round. I guess I would expect it to shoot inconsistently.

I took the plug out of a Lyman .459 sizing die, and the soft lead slug would drop right through. Obviously, the bore on that old trapdoor is a lot smaller than I thought.
If it measured only as large as .454 after passing through the bore, the free drop through a .459" sizing die doesn't seem surprising.

The question is-can a bullet that is too big for the bore result in erratic accuracy? One day I can shoot 2 1/4 inch 5 shoot groups at a hundred yards, and the next day they scatter out to 5 to 7 inchs. I pushed a half dozen bullets through that .459 sizer with my thumb so I'm going to load those and see what happens.
I would think that running your oval bullets through a sizing die would make them round...and a consistent amount over 'groove'. That alone may improve performance.

Depending on what your slug numbers really mean, you might even want to try a smaller (say) .456 or .457 sizing die.

...still wondering about that one thousandth deep rifling...

CM

[hydraulic]

Sorry for the confusion. I am unable to measure the groove diameter or the bore diameter because, as mentioned above, the trapdoor has a three groove barrel. I am in the dark as to the dimensions of the inside of the barrel. In an effort to acquire SOME idea of what was inside that barrel, I simply took a soft lead 500 grain bullet and drove it through the barrel. When I measured the slug it ran .452 in some places, and .454 in others. I ran it through a .459 sizing die and it simply dropped right through without touching the die. This tells me that my bullets, sized .459, are larger than the bore, the groove, or anything else inside the barrel . My question was; will bullets several thousands over barrel dimensions shoot inaccurately. My trapdoor shoots inaccurately, sometimes, but sometimes I get groups between 2 & 3 inches. Don answered my question by saying yes. I have also been shooting WW bullets, unsized, from an old Herter's mould which I am sure was made by Lyman. They are not round. They vary in size from .455 to .465 depending on where I place the calipres. These gave me my best group ever, 2 1/4 inches at 100, but that only happened once. I appreciate any and all comments, questions, and suggestions.

[Bad Ass Wallace]

My trapdoor has .453/.462. This I measured by placing a slugged boolit on a surface plate then lowering a dial guage to the surface, zeroing, then resting over boolit.

I use a custom 500gn mould that casts .462 and these are loaded unsized. Accuracy is consistant at 2.5MOA and a load of 68gn Wano 2P is regulated to the Buffington sight graduations all the way out to 600yards. At 800yd this load starts to fall a bit short with the sight now moved to the 950yd setting in order to strike the gong.

From your description there is something wrong with the loading technique as with a rifle grouping 2 1/4 one day and 5-7MOA the next, there must be some variable in play, different projectile, load, cleaning perhaps?

You don't mention BP, but WW is probably a bit too hard alloy to "upset" into the rifling with just BP as a propellent. Try making some boolits from a 1:30 mix and don't size before loading

Incidentally, I found a patch soaked in 'moose milk' followed by a tight fitting dry one, all that is necessary when cleaning between each shots.

Cheers
BA

[405]

Tough critters to measure eh? You can come close by lightly wrapping a slug with something like a strip of pop can metal, then subtracting 2X thickness of the metal. Takes a light touch with both the metal strip and the mic/caliper along with some judgement.... to prevent bending a chord of the metal across two edges of the land/groove edge. I always do it three times then use the average. For the bore diameter the easiest might be to take the depth of a groove (magnifying lense helps). 2X that depth subtracted from the groove diameter will give an approx. reading for the bore diameter. For sizing bullets for such a bore I usually add .001-.002 to the estimated groove diameter.... for good measure.

[hydraulic]

Problem solved! Gave the old girl a good cleaning and set her down with the rest of the stuff headed for the gun show. After expending several pounds of powder and enough lead to cause half the county to slide toward the Missouri River, I admit defeat.

[Don McDowell]

Well I hope you still have something to shoot at Alliance next month. If not let me know I'll bring you a rifle to use, you can bring 100 or so rounds of 45-70's, my extra rifle prefers bullets as cast or sized .459. Be sure to full length resize the case's so there's no problem with differing chamber sizes.

[hydraulic]

Don;

I don't know what to say. Your generosity is a reflection of the kindness and helpfulness I found at the Quigley. I shall be most obliged to you for the use of your rifle and I am now going to go down to the basement and start loading .45-70 shells. I will resize the brass, expand the cases and case mouth, drop tube 65 grs. of Swiss 1.5, cardboard wad, no compression, 500 gr. round nose bullet. I had intended to only shoot the Buffalo match on Sunday, but we will be there on Thursday or Friday. Thanks a lot, and we'll see you in Alliance. Charles Mulhair

[mooman76]

Did you try a softer alloy like BAW said? Those old guns especially the trapdoor like soft lead.

[Don McDowell]

Okydoky we'll see you on or about the 21st of August.
You'll be shooting the Taylors 74 Sharps my son Marcus managed a 24 with at the Q, first time he had ever shot the match or the rifle.

[hydraulic]

Moorman:

I ran several tests of soft bullets and ww bullets. I loaded 10 rounds of each and fired five of each, alternately, and the harder bullets allways shot smaller, that is, not as large groups.