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GregLaROCHE
11-05-2020, 10:22 PM
I slugged the barrel of a Marlin 1895 Micro Groove in .45/70 today. To measure what would be the max groove diamiter, I used my different sized NOE sizing inserts as go/no go gauges. Any reason this method shouldn’t work? It came out a tight .460”. Does that seem right? Is there a standard depth the rifling grooves were cut so I can calculate the bore diameter? I’m planning to use a PCed .460 boolit.

Thanks
pietro
11-06-2020, 02:32 PM
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In 1972, Marlin introduced the Model 1895 chambered in .45-70.

Initially, these rifles were made with 8-groove Microgroove barrels, with grooves that were .060" wide and .003" deep.

The next year this was changed to a 12-groove barrel, with the other specs remaining the same.

Again, groove diameter is spec'd out at .4587", so oversized cast bullets are called for in these guns.

All .45-70 Microgroove barrels had a 1 in 20-" twist.

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GregLaROCHE
11-06-2020, 04:19 PM
So groove diameter, not bore diameter, is supposed to be .4587? Then how did I come up with .460? As I said it was a tight .460 so maybe it is really less. Could it be the sizing die inserts?
pietro
11-06-2020, 07:18 PM
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You might be overthinking this, since the thickness difference between .459" & .460" is smaller than an RCH..... (jes' sayin' ) ;)


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wmitty
11-08-2020, 02:14 AM
My son’s 1895 measured .4593” using a soft boolit tapped into the muzzle and carefully bumped out and miked; so a tight .460” sounds believable.
17nut
11-08-2020, 02:26 PM
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john.k
11-09-2020, 07:24 AM
I checked my Marlin of 1978 against the specs provided by Marlin,and it was spot on the dimensions,exact to a tenth......this is what youd expect ,since the rifling button is made to exact dimensions,as against the variables of cut rifling ....I am/was a tool and die maker ,and the bullet was measured with an optical comparator,as well as ordinary micrometers.
Dan Cash
11-09-2020, 02:40 PM
So groove diameter, not bore diameter, is supposed to be .4587? Then how did I come up with .460? As I said it was a tight .460 so maybe it is really less. Could it be the sizing die inserts?

The slug you made in the barrel measures the groove diameter. .460 sounds about right; I would feed it .462 bullets.