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Discussion Starter #1
I've recently had shooting sessions to check the sighting of two different rifles, a Marlin 39M and a Ruger 77/22R. In each session I fired over a CED Millennium chronograph. I used two different versions of the Remington Golden Bullet for each session, the first some 10-12 year old round-nose solids and the second some newly purchased round-nose hollow points. Both batches are bulk ammo, the first 350 rounds in a mug, the second 550 rounds in a Bonus Pack.

I was very pleased with the accuracy of both guns. I was also pleased with the reliability and accuracy of both types of ammo. I fired two 10-shot strings of the old solids from each gun and three 10-shot strings of the new hollow points. I experienced only one failure to fire, in the Ruger and with the new HP. No damaged cases, no extraction problems, no squibs from either type, and each string was acceptably consistent and uniform in its velocities.

Shooting conditions were very nearly identical for both sessions, low 50s, overcast, wind from the NE at less than 5 mph. The chronograph never gave an error message and as far as I could tell was working perfectly normally. I shot all the strings off of sandbags on the hood of my Jeep.

Both the Ruger 77/22 and the Marlin 39M have 20 inch barrels. The Marlin has 12-groove Micro-Groove with a twist rate of 1:16". The Ruger has a more traditional 6-groove rifling, with the same 1:16" rate of twist.

Now, I told you all that in order to tell you this. In going over my numbers afterward, one thing stood out... the Ruger fired both old solids and new hollow points almost 100 fps faster than the Marlin. Every string showed the same thing, higher velocity for the Ruger.

I don't have the company data which came with the old solids, so I don't know what velocity was claimed for it. Remington claimed 1280 fps for the new hollow points, but I wasn't actually expecting that speed because 1) I was using a 20" barrel instead of the 24" Remington used, and 2) all ammo companies exaggerate. I did expect both types of ammo to shoot the same from both guns, though, and I have no explanation why they didn't.

Is there something obvious I'm missing? Is it because one gun is a bolt action and one a lever action?
Could differences in the rifling make that much difference? Anyone else run across this?

Spence
 

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Is there something obvious I'm missing? Is it because one gun is a bolt action and one a lever action?
Could differences in the rifling make that much difference? Anyone else run across this?

Spence
Good thread Spence. Iowaloha!

Obvious difference is two different rifles made by two different makers made on two different days with two different tools with two different rifling specifications.

I'm gonna vote that way!

Cheezywan
 

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I would hazard a guess that 12 "micro-grooves" generate just a bit more friction than the 6-groove rifling in the faster barrel...can't imagine what else it would be.
 

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Perhaps new reamer for the Ruger? I understand that the Ruger M77 has a tighter tolerance chamber as well. If the chamber and bore are on the minimum sizing then I expect you would get higher velocities.
 

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Discussion Starter #6
Perhaps new reamer for the Ruger? I understand that the Ruger M77 has a tighter tolerance chamber as well. If the chamber and bore are on the minimum sizing then I expect you would get higher velocities.
I had wondered about the pressures, too. I know that lever guns in general have a lower allowable maximum pressures, but I would assume that as long as they operate within that limit there would be no loss of pressure, no blow out of gas. I'm not familiar with the history of the Marlin model 39 action, has there ever been an issue of gas escaping that anyone is aware of? I certainly don't see any on my gun. I don't know anything about the pressures of LR ammo, but it must not be very high, and I don't think pressure loss is a factor. Higher pressure in the Ruger because of relatively smaller chamber and bore seems more likely.

I had also wondered about the friction, as broom_jm mentioned, and I think that's a likely suspect. I know that the Micro-Groove is shallower than traditional style rifling, though, so I have no idea how the actual bearing surface area compares between the two barrels.

Spence
 

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Maybe one has been shot more and has a smoother bore.


Michael Grace
 

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Discussion Starter #8
Maybe one has been shot more and has a smoother bore.
I doubt if either gun had 200 rounds fired through it before this session. Both were bought new, sighted in and put away. I can see where a slicker bore might have some small effect, but I doubt that much.

Spence
 

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Spence I am afraid that that is just the nature of the beast. I have chronographed 3 identical Ruger K77/22VBZ's in .22 LR with ammo out of the same boxes and got different average velocities from each individual gun. I have had similar results in chronographing .30-06's with the same ammo. It is just that not all barrels and chambers are identical.

Larry
 

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I vote for friction as well. A slightly tighter bore may produce a velocity difference of 100 fps. As far as pressures are concerned, I don't recall hearing anything about gas leakage in a model 39A, but I do know that later models have a hole drilled through the left side of the receiver to let gas escape in that direction in case of a case rupture. I think it was just a preemptive move to satisfy a potential safety concern. Beyond that, if HV ammo is shot in a old model 39 it can break the bolt. A new bolt can be fitted to prevent this from happening. Most pre-WWII guns were built for SV ammo only.
Thanks for sharing your findings with us!
 

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Discussion Starter #11
It is just that not all barrels and chambers are identical.
The differences must surely be extremely small, though. I can see that differences in internal dimensions of chamber and bore could make a difference in pressure, but I'm having trouble getting my head around the idea differences on that scale could have such a large effect. We are talking 8%-9% difference in velocity, and that seems a lot, to me.

Spence
 

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Discussion Starter #12
Spence I am afraid that that is just the nature of the beast. I have chronographed 3 identical Ruger K77/22VBZ's in .22 LR with ammo out of the same boxes and got different average velocities from each individual gun.
Part of my problem may be that I have an unrealistic idea of what to expect from smokeless ammo. Most of my experience using the chronograph is with muzzleloading black powder guns, and a variation of 100 fps wouldn't surprise me, there. I may be expecting more from modern ammo than it can deliver.

Well, the squirrels won't care.

Spence
 
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