Diameter and bore fit?

[unspellable]

What's the general consensus on the importance of the bullet diameter matching the groove diameter?

We are thinking of designing a cast bullet for the 7.65 mm Parabellum cartridge.  For some reason the groove diameter in most barrels for this cartridge has always been oversized.  (Not a production tolerance thing, it was quite deliberate.  The condition appears on the original DWM drawings and in the SAAMI specs.)

If we increase the bullet diameter to match the groove diameter we may run out of sufficient clearance for the case neck to expand.  This may force us to use a bullet less than groove diameter.  We'd really like to go for some accuracy.

[Leftoverdj]

Some numbers would help. How big a bullet do you need and what neck diameter do you have to work with?

If you are really serious about getting cast bullets to shoot in a particular pistol, it's quite possible that an inexpensive neck and throat reamer could be used to open the neck area the needed amount.

[Castaway]

It may be I don't understand your question, but if you have an oversize bullet, the case sould easily expand enough to wrap around the bullet and not cause any problem if the chamber can accept the cartridge.  There is a whole lot of slop in reloading brass cases.  If an over size bullet gives you problems, you can cast a softer bullet that will obturate to fill the grooves of your particular firearm and still fill the groove diameter of the barrle and accomplish the same thing.

[unspellable]

The problem is that the cartridge and chamber agree with each other.  If we expand the brass to take a larger bullet we soon run out of clearance in the neck area of the chamber.  Reaming the chamber will be a no, no, no, never, don't even think of it!  We are not trying to fit a particular pistol, this problem is common to virtually all pistols chambered for this cartridge.  (Why is a mystery so far.)  The exception being ones rebarreled with 0.308 barrel stock which is not to the original specs.

Nominal jacketed bullet diameter is 7.83 mm or 0.308268 inches while nominal groove diameter is 7.90 mm or 0.311024 inches.  There is a discrepency here.  You can't get the best accuracy with this sort of fit, especially with a lead bullet.  Reaming the chamber neck is out of the question because we are trying to come up with the best cartridge to fit all pistols chambered for the 7.65 mm Parabellum and many of them are collector's pieces, not to be modified.  If I could get a second barrel for my Ruger P89 I might think of reaming the chamber, but the Ruger P89 is not noted as an accuracy champ to begin with.

We will use a bullet with as large a diameter as the chamber neck allows without reaming the brass and that's the limit.  Since they are self loaders, reaming the case neck is not a good idea as you need plenty of neck tension to prevent shoving the bullet back into the case and the neck is rather short.  We may consider a hollow based bullet or a gas check.

Sorry, I don't have the chamber neck diameter handy.  We first thought of using a 0.311 jacketed bullet but the neck clearance is pretty questionable.

[Mikey]

unspellable:  As I recall, the bullet diameter (jacketed) on the 30 Luger is .310 - at least that is what I recall being miked out from Winchester bullets.

None the less, the importance of matching the bullet to the bore diameter is quite important.  Firing a .308 bullet down a .310 tube isn't gonna get you  a lot of accuracy but, you may wish to look at current offerings in 308 bore, as they quite often need to be sized down after dropping from the molds at a larger than bore diameter.  

The 30 Luger is going to be finicky with almost anything short of a round nose bullet, unless yours isn't picky about what it will digest (however, that bottleneck design is very forgiving on bullet designs, and you may find that even SWCs feed reliably).  Years back, some of us used to bong the gong on the silly-wet range with 30 Lugers shooting the 110 gn 308 HP for the 30 Carbine, over a full house charge of Blue Dot powders - and here I go tellin' ya 'bout the importance of bore to bullet fit (LOL).  

But also, the 30 Luger, being a jacketed bullet from the start, was designed for military purposes and smokeless powders.  The groove depth will most likely coincide with that application and not be deeper than .002, which makes me wonder if a plain based cast bullet will give you superior accuracy over a gas check, which can be driven at factory velocities so ya'll don't have to go cutting recoil springs to get those toggle tops to function.

Right off the top of my flat little head I do not recall the bore diameter of the 32-20, or the 32 S&W spls or 32 H&R, but there are some nice looking swcs that I have wondered about for general overall plinkin' and bunny bustin' with the 30 Luger.  And, as I am absolutely certain you already know, if there is a gas checked capable bullet out there, the gas checks used for 308 bullets will certainly suffice for a .310-.311 bore.  

BTW, I understand that the maker of magic cartridges, J.D.Jones, has designed a 7.63 Jones or whatever, using the 30 Luger case and a heavy bullet - he did the same with the 7.62x25mm and calls it the 7.62 JD or sumtin or anudder.  Possibly he may be s source of information for you.  

Otherwise, I would go cruisin  through a bunch of older Lyman manuals that may have pictures of cast bullets to help you with ideas.  HTH.  Mikey.

[unspellable]

The Winchester bullet has a nominal diameter of 0.309 with a max of the same 0.309 and a minimum of 0.306.  Those I have measured run very close to 0.309.  Doesn't really matter as Winchester does not make this bullet available to reloaders and the factory cartridge isn't worth a damn.  (Perhaps excepting in the Oimar 30 caliber pistols which are set up for the Winchester round.)

Cutting recoil springs in Lugers was about the worst idea anybody ever came up with.  I could go into this at length, but that's another story.

Most of the Luger's reputatuion for poor reliabilty stems from the factories deciding they could "improve" the original DWM cartridge and the erroneous notion that things can be improved by monkeying with the recoil spring.  Given a stock recoil spring and correct ammo, a Luger is as reliable as any self loader.

It has to be admitted that match grade accuracy was not what they had in mind at DWM when they decided on bullet and bore dimensions.  We are stuck with the existing chamber and bore and have to come up with the best bullet for it.  The heck of it is that the Luger is quite capable of match grade accuracy if you can get the ammo right.

Years ago I ran semi-wadcutters and full wadcutters in my 7.65 Luger with perfect reliability.  Never had a jam the entire time I owned the pistol.

A friend has been running cast 9 mm bullets at full velocity without leading problems.  He has been moly coating them.

All this is a fairly major project.  We have been doing a lot of research to seperate the hearsay form the facts.  Including reduced recoil springs.