OAL v pressure v velocity v powder

[mag41uk]

Maybe not the best title but I will expand.

I decided to chrono loads for my No4 and K11 yesterday and the results werent what I expected!
303 40.8gns N140 with PPU 174gn bullet. Bullet seated out far enough that they wont fit in the mag.
Now I expected to see around 2450 duplicating factory load. The result, around 2270 fps!
This is lower than the Viht starting load of 38.4gns and 2380fps.

My K11 is loaded with 155gn scenars and 48gns of N150 and I got 2660fps. I expected about 2560fps.
Viht list a max of 49.7 for 2674fps. The bullets for this are also seated past recommended length.

All charges trickled via Targetmaster into Lyman scale.Brass neck sized only.

As I was shooting at 1000yds today I gave the 303 a miss but used the K11.
Missed with the first couple of rounds ( I already have settings for 900yds) but with the aid of a spotter got on target.
Out of 40 rounds had a couple of vs + 5s + 4s + 3s and the rest off to right,due to slight and variable crosswind.
Chuffed to bits to say the least!

Anyway,thoughts on my chrono results?

Tony

[Sandgroper]

The longer OAL will lower the pressure (and hence the velocity) of the round.


from http://bulletin.accurateshooter.com/201 ... -to-ogive/

Effects of Seating Depth / COAL on Pressure and Velocity
The primary effect of loading a cartridge long is that it leaves more internal volume inside the cartridge. This extra internal volume has a well known effect; for a given powder charge, there will be less pressure and less velocity produced because of the extra empty space. Another way to look at this is you have to use more powder to achieve the same pressure and velocity when the bullet is seated out long.

[Steve E]

Mag41uk, on what reasoning did you think that 40.8 grns of N140 would duplicate the factory load of 2440 fps?
The test barrels that Vhit' use are not the same as your No4. They are probably 'true .303 barrels (.303 bore with a .311 groove) as opposed to a military spec barrel that could have a bore diameter of .303 to .310'' and a groove diameter that could be as big as .315''
Have you slugged your bore to determine its dimensions?
Trying to duplicate a MK7 velocity using a boat tail bullet in a modern case that is lighter in construction is a lot harder than you think. Even when you duplicate the velocity, the rises will be different because you are using a boat tailed bullet as opposed to the original flat base bullet. The boat tail bullet will have a flatter trajectory than the flat based MK7.

[Dougan]

I decided to chrono loads for my No4 and K11 yesterday and the results werent what I expected!
303 40.8gns N140 with PPU 174gn bullet. Bullet seated out far enough that they wont fit in the mag.

If you're seating them that long then they're not really a copy of the factory round - The Lyman book gives a typical .303 round being 3.075", but of course it depends on the bullet...

...when I started on .303 I remember measuring the OAL on an average (was some variation) PPU round at 3.045"...so when using the PPU bullet I aim for 3.050, and am happy with anything from .045 to .055.

As SG said, the deeper the seating (so shorter OAL) the higher the pressure, and with just 39 gn of N140 I'm getting very good results at 200/300 yards - I've no idea of the velocity (would love to chrono them...), but I do know they start to drop off at 600, so would need more powder for the ranges you're shooting...

[Alpha1]

I use 41g of Vitavouri with a 174g bullet and seat the bullet to 3.000" they work fine in my No4.

[mag41uk]

Thanks for replies chaps.
My No4 Mk2 is a UF55 and its does slug at 0.311.
I understand about increasing case capacity affecting pressure and velocity.
I dont understand why there is such a deviation in my 303.
Vihts data is for a 174gn SMK (in a test barrel @ 0.311) which I also use and they chrono the same.
I am loading 1 grain down from max listed which is why I made a reasonable assumption I would be near 2450fps.
As it happens I do shoot this load at 900yds.

I still dont understand the K11 loads.This rifle slugs at 0.308.

And FWIW, I have been reloading over 30 years now but still learning!

Tony

[Dougan]

Hi Tony - I wasn't going to comment further as I've only been loading a few years; but as I've had some similar issues with seating depths quoted in the Vhit manual (which for many loads are longer when compared to the other manuals...Lyman, Speer, Hornady etc.), I'll share what I found (whether it's right or wrong) and you can draw your own conclusions as to it's relevance...

When I started loading for my Swedish Mauser, I used the starting load and the (very long) OAL quoted in the manual, which left very little of the bearing surface of the bullet (SMK) inside the case neck - This resulted in ejector marks on the case head, but not because of over-pressure, but due to the bullet releasing before the case had sealed against the chamber, causing it to push backwards - The marks were also accompanied by sooting down the case...are you getting any sooting?

I the end I ended up with loads that are actually below the Vhit minimum for powder (I checked that it was a safe volume) but with the bullets seated well in to get the cases to seal properly - My 6.5 seems to like PPU bullets more than SMKs (don't know why), and when I use them I seat them right in just the same as the PPU factory rounds.

And that leads to another point; using SMK bullet data with PPU bullets...

...the recommended OAL in the Vhit manual for a .303 174 gn SMK is 3.071 - if you look at how much of the bullet's bearing surface is inside the case neck at this length it's not too bad (but still not enough in my opinion) - Now if you compare the PPU bullet to the SMK it has a longer boat-tail, so when seated to the same recommended length as the SMK it has even less of it's bearing surface in side the case neck - Also (and same with 6.5s), where as the diameter on SMKs is always bang on, i.e. .311, the PPUs can be just under.....and this combination of less diameter and less bearing surface could be giving a significantly reduced neck tension...

When you say that they're too long for the mag on your number, what is their actual OAL?

[Sandgroper]

Thanks for replies chaps.
My No4 Mk2 is a UF55 and its does slug at 0.311.
I understand about increasing case capacity affecting pressure and velocity.
I dont understand why there is such a deviation in my 303.
Vihts data is for a 174gn SMK (in a test barrel @ 0.311) which I also use and they chrono the same.
I am loading 1 grain down from max listed which is why I made a reasonable assumption I would be near 2450fps.
As it happens I do shoot this load at 900yds.

I still dont understand the K11 loads.This rifle slugs at 0.308.

And FWIW, I have been reloading over 30 years now but still learning!

Tony

My thoughts are that the increase in the OAL is the main culprit. What was the OAL of your ammo vs that in the Viht reloading manual?

[Dombo63]

This is interesting to me as I have recently begun looking at reloading 303 with Lee kit. I measured the OAL of my S&B and PPU factory rounds and found them to vary between 3.055 and 3.06, while HPS Target rounds were almost bang on at 3.07 and showed the most consistency. They also have a bit more poke than the S&B rounds - shooting a couple of magazines prone wearing just a T-shirt does smart a bit towards the end.
I have only so far played around with neck-sizing and seating using my once fired S&B brass to get an idea of tightness and depth. The brass seems pretty rubbish a holding a tightly seated bullet so I intend using just the PPU brass when I do it for real.

[Laurie]

There is a great deal of misunderstanding here. COAL alone within reason doesn't affect peak chamber pressures - it does in large measure though when taken into account with freebore. That is if the SAAMI or CIP COAL for a cartridge is 3.00" but the rifle has long freebore (or badly worn throat which is often the case with surplus military rifles) so that the bullet is sitting just off the lands at say 3.300" COAL, a given charge will only produce minor pressure variations at either COAL. That assumes the charge suits the case capacity and isn't massively compressed at the shorter COAL.

Why so? Internal ballistics says the combustion chamber capacity is the key factor - that is the internal volume of the expanded and obturated case + any space between the case / chamber and bullet base. The bullet leaves the case and enters the leede purely under primer gas pressure - as anybody who has forgotten to charge a case during the loading process soon finds out! In a worn throat or long-throated barrel, the bullet is well forward LONG before the charge burn produces peak pressure, so the COAL on loading isn't the key factor.

QuickLOAD COAL changes are predicated on the basis of that ocurring and when you input COAL assumes that the bullet isn't making a major jump before it's engraved by the rifling.

This plus (as previously noted) worn or oversize barrels plus slack chambers is one reason why many military rifles produce much lower pressures and velcoties than loading manuals predict as well as poor case obturation resulting in badly sooted case bodies. It's not just wear - many government rifle factories obviously didn't change the throating / freebore on moving from long round nose heavy projectiles of the late 19th / early 20th century preiod when the ammo chamged to lighter pointed bullets. In such cases, standard COALs with the later bullet would see a lot of freebore and hence bullet jump even with an unworn barrel.

Providing the charge burns cleanly, MVs are acceptable and ES values reasonable (the definition of 'acceptable' here being an interesting issue in itself compared to what is expected of match grade long-range handloads in a quality modern rifle or even more so gunsmith built custom rifle) AND precision is good with acceptable groups, all this is frankly no bad thing for the life of a maybe irreplaceable barrel in a good historic arms rifle.