I am planning to do a test on different diameter bullets, in 3 different calibers.

.375, .308 and .243.

I have already started with the .375.

I have shot 3, 5-shot groups, with bullets of 0.375, 0.376, 0.374, 0.373 and will continue with larger and smaller bullets until I reach a point where I notice something really different.

I am not going to tell you my results so far, but I would like to hear your opinions of where you think I might start noticing major differences.

How many 0.001s difference, below and above the normal caliber.

Your feedback would be much appreciated.

Are you interested in deviations in accuracy or penetration, or ?

I am only checking the velocities and accuracy.

These are the only relevant criteria, I think, we should be looking at.

What I wanted to find out is how much of a difference in the diameter before one could see any difference.

lots of interesting variables... oversize will cause higher pressure issues, undersize will cause less spin, stabilization...

my guess is .004 oversize and .009 undersize... look forward to your results... thanks for your opportunity...

I think your actual bore diameter is a major variable here, as measured as opposed to nominal... depending on how much rifling you have left, it will change.

ETA

I suspect accuracy will go to pot once you have only a couple thousandths of rifling engagement.

Velocity will be much more tolerant of differences until you get substantially over or under.

Yep, gotta know what Saeed has found by slugging his barrel groove diameter,
and what is his bore diameter determined by the largest pin gauge he can slip into his bore,
surely within 0.0005" tolerance.


Homologated barrel rifling diameter MINIMUMS for .375 H&H:

CIP: Bore 9.30mm = 0.366", Groove 9.55mm = 0.376"

SAAMI: Bore 0.366", Groove 0.376" ... Same as CIP.

Bullet diameter MAXIMUMS:

CIP: 9.55mm = 0.376" ... No minus tolerance specified

SAAMI: 0.376" -0.003" ... A minus tolerance is specified, minimum bullet diameter 0.373" tolerated.

McGowen specs for .375-caliber rifle: .368" bore and .375" groove diameters.

For monometal copper bullets, I am going to be a smartass and say you won't see much difference until you get to 0.002" bigger than groove diameter or 0.004" smaller than groove diameter.

If only .002" smaller than groove diameter, accuracy will be good but velocity will be decreased for a fixed charge of propellant.
Accuracy and velocity will really drop off past 0.003" smaller than groove diameter.

Best accuracy will be at +/- 0.001" from groove diameter, with common bullets of large bearing surface.
Walterhogs have such a tiny bearing surface, they might be best for velocity and accuracy at +0.002" or greater above groove diameter.

I might be just blowing smoke.

Accuracy across a wide range of loads will be better with .374. However, that is my expereinces but based on lead core bullets but I thinkit wil also appy to mono bullets.

Very interested. My 500 Jeffery is very accurate with the Barnes 570g TSX which I think is a .509" diameter.

My guess is that you will see mono-metal bullets have a much lower =/- tolerance
than lead core bullets.

I've seen many rifles that just don't like solid copper bullets but would shoot lead core bullets accurately.

I am only shooting one load.

Maximum charge specified in the vihtavuori book for VVN 140.

I will check the bore in this rifle and posted.

Assuming the groove diameter is a true 0.375", I would doubt that one would see much, if any, difference between a 0.375" diameter bullet and those of plus or minus 0.001" in diameter.

An old adage, which may date from lead bullet days, holds that one should shoot the largest diameter bullet in any nominal caliber that will chamber in one's rifle. That's for the sake of accuracy. Velocity would suffer, I would think.

Saeed, I wouldn't have clue, but I am very interested in what you/we can learn. Thanks, Brian

Bore diameter of this rifle is 0.367

quote:

Originally posted by Michael Robinson:

Assuming the groove diameter is a true 0.375", I would doubt that one would see much, if any, difference between a 0.375" diameter bullet and those of plus or minus 0.001" in diameter.

.001" makes more difference than you think. However, my experience is with undersize barrel and specifically .3065 for 30 and .357 for 35 and with multiple 358 STAs.

When I first got a 358 STA many years ago our main match barrel maker said to with a .357 barrel. This was based on him using .3065 for 30 calibre, both button and cut rifling. The reason he had done .3065 was because of what is called Full Bore target shooting in Australia. Ammo can't be reloaded and they had to use the 144 grain 308 military load. I think it was some sort of selected ammo. Since the reloads could be used they fiddled about with chambering, throat etc. and barrel diamter to get the best result. The 3065 barrel were noticeably better and undoubtedly one reason being the very short bearing surface of the 144 grain boat tail bullet.

He then tried the .3065 barrels in other 30 calibres. Results in 300 Wby were very good.

The different 30s and 358 STAs were also on switch barrel bench style rifles, with Rem 700s, Jewell triggers etc.

The stand out difference was the accuracy across a very wide range of loads. The accuracy with the best load for each was much the same. But a world of difference when tried across a wide ranger of loads.

0.375 bullets.

Average of 3, 5-shot groups 0.7813" velocity was 2375 fps.

This is as with all loads, is 66 grains of VVN 140, which is slightly more than they show as maximum, they had 65.9 as max.

0.376 bullets.

1.1280", 2386 fps

0.374 bullets.

1.5487", 2363 fps.

0.373 bullets.

1.4053", 2349 fps.

Not really sure if we can read anything into this.

Close enough to be what one might get at different settings.

And we continue!

0.371 is where things went to hell.

Two groups only, 3.654" and 2.804"

Velocity was 2334 fps.

Let us see how far we can go in larger diameter.

quote:

Originally posted by RIP:
For monometal copper bullets, I am going to be a smartass and say you won't see much difference until you get to 0.002" bigger than groove diameter or 0.004" smaller than groove diameter.

If only .002" smaller than groove diameter, accuracy will be good but velocity will be decreased for a fixed charge of propellant.
Accuracy and velocity will really drop off past 0.003" smaller than groove diameter.

quote:

Originally posted by Saeed:
0.371 is where things went to hell.

Two groups only, 3.654" and 2.804"

Velocity was 2334 fps.

Let us see how far we can go in larger diameter.

AH SO !
Muchly as I predicted on the decreasing diameter bullets,
IF your groove diameter is 0.375" to go with that 0.367" bore diameter.
We assume groove diameter is .375" or has Saeed measured that ?

I was thinking it would be interesting to make the bullets slick-sided like the original Barnes X-Bullets,
instead of like the fancy-drive-banded Walterhogs.

With full-bearing shafts, the bullets might not go to hell so quickly as diameter is decreased,
but might go to hell more quickly as diameter is increased.

I am going to do a more comprehensive test on the 308.

We had a discussion here and a suggestion was made that we should test both copper and brass bullets.

The "free-machining" brass will make some pretty bullets.
Brass is slicker, harder, more brittle on delicate drive bands.
Less deforming on impact of an FN solid nose,
but more likely to strip the driving bands on poor fits to rifling.

Wide bands or full-bearing shafts, or the skinny Walterhog bands ?

The grabbier, less brittle, softer-but-tougher copper is better with those skinny drive bands.

Pictures of bullets, please.
A sample or two showing the bearing surface pattern is important for interpreting your results.

These are our normal Walterhog solid bullets.

I will post photos.

Check your Whatsapp.

I have sent you photos.

Went up to 0.380.

At 0.381 I found it difficult to seat the bullets in the Redding dies - the necks were touching the inside of the die as the bullets were seated.

So that concludes this test, I will post photos.

Here are examples of the bullets Saeed is making:

.375-ish-caliber of XXX-gr weight:


.308-caliber of 160-gr weight:


.XXX-caliber of XXX-gr weight:

For testing different diameter bullets in the 308, I made a few of our standard 160 grain bullets they shoot very well in the 300 Winchester magnum.

I fired a few in my test rifle - a Robler TITAN.

Shot terrible.

I know the rifle shoots well, but with these bullets groups were about 2 inch!!??

I went and modified the bullet, by adding another drive band further back on the long boat tail.

First two shots went into one hole!!??

5 test shots looks like 0.3-0.4"!!

The original bullet on the left.

Nothing wrong with it, as it shoots well in other rifles at higher velocity.

I have never seen this before.

You also shortened the boat tail and minimized the other driving bands when adding the extra band, making total bearing surface about same, eh ?

It appears that the skinny driving bands do better when major diameter is significantly greater than groove diameter.

What are the minor diameters (between bands) on the "375" and "308" bullets, please ?

And what sort of throat is on the Robler, compared to the usual .300 WinM., pretty please ?

Funny thing about the SAAMI .300 WinM., it has a wide-based, leade-only throat like on the SAAMI .458 WinM., only not so acute a leade angle.

.300 WinM.: 0.3150" diameter start, 1*26'37" leade hemi-angle.
Maximum bullet: 0.3090"-0.0030"
Minimum bore: .300"
Minimum groove: .308"
Twist: 10"

.458 WinM.: 0.4690" diameter start, 0*29'30" leade hemi-angle.
Maximum bullet: 0.4590"-0.0030"
Minimum bore: .450"
Minimum groove: .458"
Twist: 14"

Both are capable of remarkable accuracy,
much to the chagrin of Art Alphin.

Well, like lots of things here, things did not go as planned.

A friend borrowed the Robler, as he did not want a Remington 40X I offered to lend him.

So I am going to use a Remington 40X, which is a very accurate rifle with our lighter bullets.

140 grains.

Fired 3 shots to try and all I can see is one hole!!

I will make this test of 5, 5-shot groups.

I will do both copper and brass bullets.

So start guessing what might happen.

They will all be 1-hole-5-shot groups until the Walterhogs get to 0.311" and greater in diameter
or 0.305" and smaller in diameter.