284 Shehane - what real gain ?

[pjifl]

I am trying to reconcile the figures from Quickload applied to stretching the 284 WIN case to a Shehane against with what was expected.

All the theory I see suggests well under 20 fs gain in velocity FOR THE SAME PRESSURE IN THE SAME BARREL ASSUMING THE SAME PROJECTILE SEATING DEPTH ETC. OF COURSE THE POWDER WILL BE INCREASED SLIGHTLY UNTIL MAX PRESSURE IS THE SAME.

In fact the figures suggest there is far more in velocity to be had by moving the projectile forward as much as possible.
Or the use of Moly would also allow more powder giving far more velocity gain.
Or the velocity gain from a barrel which is peaking will again be greater than that from the Shehane case.

It would almost need a rigidly controlled experiment involving actual pressure measurement to definitively find out because any change of V would be very easily masked by other factors.

But I would be interested in any actual measurements of V increase people have actually observed. Even if other parameters are not exactly equal.

Peter Smith.

[RDavies]

I haven't done anything scientific, but my finding are different to what I hear over seas where they get an extra 100 fps.
I have only ever used a Shehane, never a straight 284, so no tests. Nearly everyone I have questioned in Australia is running very similar velocities whether they are using a Shehane or straight. Your prediction of an extra 20 fps seems pretty accurate to what I would expect.

[AlanF]

Peter,

I agree that its probably very little difference, and to be honest I recommend straight 284 to anyone who asks. I justify it to myself by saying it will help to keep pressures down (for a given velocity), and that should help to prolong primer pocket life. Also, there does seem to be some truth that the Shehane is less likely to have sticky extraction. I already do some of the things you mention such as moly projectiles, have a very long freebore and jam 0.015", and additionally go for as long a barrel as practical. With all those things working together, I can maximise brass life for a given velocity. I must say the Redding S-Type FL die sold by Bill Shehane works very well in my experience. He told me its actually designed to squeeze the full length of the case by 0.002", whereas most FL dies squeeze the shoulder more. This could be a clue as to why extraction is never sticky.

Alan

[Seddo]

Doesn't the improved shoulder help reduce case stretching?

[AlanF]

Seddo,

Its not an improved shoulder - the angle stays the same, and at 35 degrees, cases don't grow on either. The only difference of the Shehane is the reduced body taper - the shoulder diameter increases by about 0.010".

Alan

[pjifl]

As Alan said, the shoulder angle is essentially the same. I say essentially because I cannot find any 'official' specs for the Shehane and some figures I see seem to headspace it slightly differently to the 284 WIN while others don't. In one case the angle changes by about 1-2 degrees which is essentially the same anyway.

Nor is the shoulder very wide in comparison with a smaller caliber. In fact, when one draws shoulders of a larger cal such as a 308, it becomes obvious that the shoulder is quite short so a change in shoulder angle has far less effect than say a 6BR with a much longer shoulder. Quite obvious, really, but hits home when you compare drawings.

Extraction of a 284 Shehane should be roughly equivalent to the 308 - the body taper is the same for the 308 and 284 Shehane because the standard 284 case has more body taper than most modern cases. Which makes it more attractive to wildcatters.

I don't think I will be building a 284 Shehane - just trying to understand it better.

Peter Smith.

[AlanF]

Peter,

Now that I have dies and reamer for the Shehane, I probably won't be building a straight 284 . If I could muster up the enthusiasm, it would be a modified RSAUM, with the shoulder pushed back to about the same capacity as the Shehane. That should run the 180s at 2950 without brass life problems. But I'd imagine forming the brass would be a multi-stage process and a PITA.

Alan

[pjifl]

Alan wrote

If I could muster up the enthusiasm, it would be a modified RSAUM, with the shoulder pushed back to about the same capacity as the Shehane. That should run the 180s at 2950 without brass life problems. But I'd imagine forming the brass would be a multi-stage process and a PITA.

Interesting comment. Do you know anyone who is doing this ?

The Saum is not a huge capacity to start with and I would think twice about reducing it that much.

Roughly I make it that

284 WIN -------- 0.272
284 SHEHANE -- 0.279
7 SAUM ------- 0.301 which I would think very close to the 7 WSM
300 WSM ------- 0.326

While these may not be exact, I would think they would be accurate to each other.

I think the benefits from running something standard are significant. Even if you have reamers and dies, in the event of a disaster, you would probably be able to buy cases to start again quickly although they may be worth a fortune.

Peter Smith.

[AlanF]

If the current brass supply problems for the SAUM continue, neither will be a good option.

[pjifl]

I have 2 barrels set up for a straight 284. One neck turned and another no neck turn.
One shoots very well the other extremely well.

I can always fall back on these since I have a lot of 284 brass and do not run hot loads.

The SAUM, if I do it, will be for special occasions.
But no rush. Many other projects.

Peter.

[aaronraad]

It's all about the brass!

[DaveMc]

Alan - I wouldn't bump the SAUM back either - it is also a rebated rim case and suffers the same issues as a 284. It purrs at 2950 on 2209 as is and brass lasts extremely well. Push it at higher pressure and life expectancy of brass is greatly reduced (not so bad on 2217 etc)