For the benefit of the group, the original question was: How do your .458" 350 and 400 grain FN bullets work in a Marlin 45-70 with the "micro-groove" rifling?

I didn't really get the answer I was looking for, probably because I wasn't specific. My bad.

I have 2 concerns:

1. "Stripping" of the small bearing bands on your bullets in the shallow grooves of the micro-groove rifling. It's well-known that micro-groove rifling tends to strip softer (lead) bullets; your bullets aren't as soft as lead, of course, but could the smaller bearing bands (and therefore a smaller "bite" when the bullet takes the rifling) + the shallower "micro-grooves" = stripping?

2. Land diameter - The groove diameter of a micro-groove barrel is correct, .458" or so; it's actually the lands that are shallow, so that land diameter is greater. Doesn't that mean the sub-diameter section of the bullet (any bearing-band style bullet, not just yours) won't be in contact with the lands? Am I correct in assuming the land-diameter section is sized for a certain depth of groove?

Fat,
CIP as well as SAAMI specify the minimum land and groove diameters of all calibers. Manufacturers are allowed plus 30 micron and in some cases plus 20 micron on these dimensions. All my HV and FN bullets are configured for the body of the bullet to run on the lands with a ten micron interference and for the driving bands to conform to the groove diameter with a twenty micron interference. The Sako microgroove barrels I have measured conformed to these specs as did as a couple of polygon barrels I measured. I have not measured a Marlin microgroove barrel, but would be surprised if they do not conform to the specs.

I think a first step would be to confirm with Marlin the dimension specs of bore/groove they manufacture to and to take it from there. Would anyone have their contact information handy?

30th Nov.
Marlin does not have an e-mail facility on their website. Fat would you write them at:
Marlin Firearms Company,
P.O. Box 248,
North Haven, CT 0647.
and ask the question as we discussed it above? It will be faster than the cleft stick mail from Africa.

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Gerard Schultz
GS Custom Bullets

[This message has been edited by Gerard (edited 11-30-2001).]

I looked at Marlin's website, too, and I noticed they have dispensed with the microgroove barrel in all their 45-70s (it's in their FAQ), so for me at least, it's not an issue. I'll contact them anyway.

[This message has been edited by Fat Bastard (edited 12-01-2001).]

Gerard: you quote the standard as a 'minimun'. Yet there's no limit as to how large they can be. I've slugged 45 cal barrels that went from 4565 to 461- and not all of them Marlins. A 458 bullet in one case would swage to groove dia and in the large case ride partial land engravement.

Consider total land height. Say the average 6 groove 45 cal barrel has lands which are .004 high. 6 times .004 gives .024" for a value of total land wall/height for gripping the bullet. The Marlin is a 12 land barrel- assigning a value of 2.7 thou of land height, that gives .0324" of land height-- or 26% more land driving surface to grip the bullet. The driving side of the land does the work- keeping the bullet from slugging/slippage. The Marlin having 26% more driving side area surely should grip any jacketed bullet fine.

aladin,
There is a maximum allowable dimension. CIP lays down the specification as a minimum bore or groove dimension and allows between 20 and 30 micron up from that spec, depending on caliber. For example, a 7mm Rem Mag barrel is specified as bore - 7.02 mm and groove - 7.24mm, plus 30 micron. If a barrel measures more than 7.05 and 7.27, it is over spec. 20 micron is 0.0008" and 30 micron is 0.0012".

The spec for a 45-70 is 11.43mm (0.45") bore and 11.58 (0.4559") groove with a 458 Win Mag spec at 11.43mm bore and 11.63mm (0.4579") groove. Land height on a 45-70 should thus be 0.0059" and on a 458 WM 0.0079".

Specs such as those put forward by CIP bring uniformity to the industry and I am amazed at the number of manufacturers of bullets as well as barrels that seem to disregard them. Fortunately, in SA the specs adopted by the SA Beureau of Standards from the CIP standard, forms part of our enforcable legislation. Out of spec bullets, components and barrels are therefore technically prosecutable under criminal law. I do not know if it has ever been done, but it is there.

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Gerard Schultz
GS Custom Bullets

Gerard: CIP is an agency in your country or Europe- I'm not familar with it?

A large bore of course will produce less pressure that a tight one. Keeps the manufacturer out of any possible litigation a tight one might produce. And the vast majority of the shooting public doesn't know the difference anyway.

CIP is a European organisation. The SABS in South Africa adopted the CIP specs as well. Very few European countries have not adopted these specs and as far as I know, the Americas are not signatories to these organisations.

As far as liability is concerned, it occurs to me that if something is out of spec over or under, the user has a case against the manufacturer. I would rather have a tight barrel and chamber as it is less likely to cause problems than a loose one. One thinks of case necks splitting and case head separation and such.

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Gerard Schultz
GS Custom Bullets

Sorry if'n I seem ignorant, but I didn't catch much of this post. I just bought a Marlin in 45-70. Are cast bullets in this a problem?

Cast bullets of proper diameter will work with either microgroove or Ballard rifling. The key word is SIZE in cast bullet shooting and not rifling types. Hardness has not a whit of merit either. The Micro groove rifling was trash talked to death by Neanderthal's that didn't have a clue what they were implying. Of course these same Neanderthals may now be helping sell new Marlin rifles.

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"Use A Big Enough Gun!"

I just called Marlin. I'm not sure they understood what I was asking for, and they seemed kind of suspicious as to why I wanted to know. The truth didn't help, either ("Well, there's this bullet manufacturer in Africa, see..." )

Here are the numbers:

Ballard rifling - groove dia = .456 - .458
land dia = .450 - .452

Micro-groove - groove dia = .458 - .4595
land dia = .452 - .4535

If the Marlin guy and I understood each other, the depth of the grooves is the same for both types (0.006). This does not square with their statement that the grooves on the micro-groove are shallower.

This is all academic, since they no longer use micro-groove in their 45-70.

Here is a wild stab in the dark, so take it with a dose of skepticism until someone says yea or nay, but when they say the grooves are shallower I think they might be referring to the width of the groove and not the depth of the groove. If you have more lands around a circumference they will of course have to be thinner. The only guns I have with micro grooves are 22's, and I've never seen any evidence of stripping on them and they give good accuracy, but then Springfields with 2 groove rifling are supposed to be accurate shooters too.

Personally I wouldn't be concerned with shearing lead, after all even though the rifling is narrower there are more of them.

<<Hardness has not a whit of merit either.>>

That statement is simply not true, disregardless of rifling type. Soft bullets will not hold the rifling in any type of land/groove style to the manner in which hard cast will. And at what speed and chamber pressure they start to slug/skip over the rifling depends on the barrel quality, the original size of the bullet and it's hardness, lube, and likely many other variables.

I like micro's very much. As with any barrel shooting cast- the keys are rear gas seal to keep gases from coming up the bullet sides, fitting/filling the throat again for seal and alignment, and a smooth, uniform bore free of jacket fouling.