S&W 915 Undersized Barrel


January 20, 2014, 08:49 AM
New to the forum and looking for some expert advise or wisdom. I bought a S&W 915 many years ago from friend, but I have never been able to get it to shoot well, meaning group. I decided this weekend to slug the barrel, if for no other reason than to just do it. I was surprised to see the slug measurement come in at 0.348 to 0.349 depending on where I measured on the slug. Is it typical for a barrel to measure this much under size? Is this an issue for shooting normal diameter 9mm bullets that are 0.355 to 0.356 in diameter? Does this pose any other issues besides the normal ones, if I want to shoot lead cast bullets? I don't shoot the gun that much, because it frustrates me why it won't group well. The barrel has no leading in it now, maybe becasue I don't go with max loads on it, but maybe this undersized barrel is the issue with the grouping problem. Any thoughts or suggestions.

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January 20, 2014, 08:38 PM
It can be difficult to measure the slug properly depending on the type of the rifling, chances are it is within spec. Generally, an undersized bore will not cause an accuracy problem since it will swage the bullet down to fit the bore. Is the accuracy poor with different bullet weights and styles?

January 20, 2014, 08:42 PM
how many grooves are there? If the grooves are odd numbered, then you can't use calipers or mics to measure.

One other reason it might be that small...does it actually have rifling?

January 20, 2014, 09:23 PM
Thanks for replies. Thus far I have not found a good combination of bullet weight or powder. I have mostly tried 115 and 125 gr with Win 231 and Silhouette. I can count 10 separate rifling lines around the slug, so I can find land areas on either side of the slug in which to measure. The slug I made is a cylinder about a half inch in length, with a taper on one end. I measured the pure lead slug before I pushed it through and it measured 0.359 inches, but when it pushed out it was now 0.348. In speaking with S&W represenatives, they say it should be 0.354/0.356, but offered nothing beyond that. I am considering buying a new barrel to see if the grouping would improve with that, but don't want to spend the money if that is not really the issue. I have just never heard or seen a barrel that much off from what the specs say it should be.

January 20, 2014, 09:24 PM
If there are 10 lines, there are 5 grooves, so you can't measure with calipers.

January 20, 2014, 09:29 PM
I think you will find your S&W 9mm barrel has 5 lands & grooves.

That means you cannot slug it and measure it using conventional measuring instruments.

In short, it does't measure .348" - .349" as you believe.

I betcha it measures .356".


January 20, 2014, 10:02 PM
Okay, I was hoping there was something that I was doing wrong. Would a gunsmith be able to verify the bore?

January 20, 2014, 10:12 PM
Couple of questions:

1. What do you consider a good "group"? IE, at what distance are you testing it and what results are you getting?

2. Have you had someone else (preferably someone whose marksmanship skills you trust) shoot it? Depending on your own handgun skill it can sometimes be hard to tell when the gun has a problem and when you need more practice.

January 20, 2014, 10:25 PM
Would a gunsmith be able to verify the bore?A good gunsmith could.

The other 85% who are 'parts replacers'?
Probably not!

But it would be next to impossible for a S&W of that age to have a goofy bore size.

I think you are barking up the wrong tree.
The 915 was never intended to be a target pistol.


January 20, 2014, 11:00 PM
Okay now that I thought about it for a few minutes, I agree that with an odd number of grooves I can not measure land to land, 180 degrees across from each other. However, if I did mesasure the diameter, i.e. 180 degrees across I would be on a land on one side and a ridge on the other, where a groove was. This should if anything give me a little larger measurement, but never a smaller one. The odd number of grooves cannot yield a smaller diameter slug.

January 20, 2014, 11:12 PM
Yes, it can, and it will.

If you measure a slug from an even number of lands & grooves?
You are measuring to the tops of two groves in the barrel. (lands on the slug.)

If you measure a slug from an uneven numbers of lands & grooves?
You are measuring from the top of one land to the bottom of one groove.

Typically that will be .003" to .004" smaller then actual bore diameter to the tops of opposing lands in an even number of lands & grooves.


January 21, 2014, 01:22 AM
Just use a 108 degree anvil micrometer to read the slug. http://www.shars.com/products/view/1816/021quot_108_Degree_VAnvil_Micrometer

On a 3 grooved slug use a 60 degree anvil micrometer.

January 21, 2014, 08:20 AM
Okay sounds like I misunderstood what points I was suppose to measure to. I was trying to get land to land, but sounds like the correct measurement is groove to groove. Then I agree an odd number of grooves, will yield a low measurement, unless the special micrometer (as previous pointed out above) or other more compliated methods are used.

As far as the grouping I am seeing, well the best I have obtained is maybe 5-6" diameter at around 20 feet, and this was maybe once. I am by far no marksman, but I have shot 3" groupings with other pistols, just could never get this one down with all my tinkering. I think I will take the suggestion to have true marksman try shooting it. I have a buddy that has shot in tournaments for many years and through his time in the Navy. He shoots a 45 at the range and he can drive nails with it, so will see if he can work up a load that will work with the gun.

January 21, 2014, 12:39 PM
Have you shot from a Ransom Rest or some kind of rest? Otherwise, it's going to be difficult to eliminate the human factor when trying to determine groups with a pistol.

January 21, 2014, 04:27 PM
I talked to Ruger about my sr9 barrel being over sized and they told me they size to .350 + or -.004. That gives you a range from .346 to.354. Seems like a lot of difference.

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