extrapolating loads...

[bald1]

I had previously found that 46 grains of VARGET with GM210M primers in Federal Gold Match .308 WIN cases (OAL 2.800") was very accurate using home moly coated Hornady 168 grain and Sierra 175 grain bullets.

Shadows have killed attempts to get valid chronograph results for the 175gr load but we were able to get solid readings for the 168 gr load (2695.6fps with a 17.5 Sd out of a 20" tubed Savage 10FP).

Time doesn't look like it'll permit more attempts to chrono the 175gr load so we started to look at extrapolating. Everything is exactly the same except for the bullet so my thoughts are that the 175 load ought to have somewhat greater muzzle velocity, say around 2720 fps from the same rifle. Is my logic correct? Anyone have experience extrapolating like this?

-=[Bob]=-

[This message has been edited by bald1 (edited August 24, 1999).]

 

[bfoster]

Measure it. There are just too many variables other than bullet mass (especially between differing brands) for accurate prediction of velocity based on this alone.

 

[Mal H]

bald1 - bfoster is absolutely correct, it is impossible to predict one bullet's velocity based on that of a different bullet. You might be able to guesstimate to within a 100 fps range if you're lucky.

To answer your question, however, your logic is backwards. In general, given the same amount of the same powder, a heavier bullet of similar type will have a lower velocity than a lighter one. This may not hold up if one is moly coated and the other isn't. Moly will tend to make a bullet a little slower, believe it or not. It seems like it should be faster, but the lower friction yields a slightly lower max pressure, hence lower velocity.

 

[bald1]

bfoster.... we want to chrono the load but as stated circumstances may not permit in the short term.

MalH... I too thought logically the heavier bullet would be slower but looking at the reloading manuals seemed to indicate otherwise. For example Sierra (anniversary edition) shows 40.2 gr of Varget necessary to push a 168gr to 2400fps whereas only 40 gr is needed to push a 175gr bullet to 2400fps. The newest Hodgdon manual (#27) shows 42gr of Varget giving 2520fps for a 168 bullet and 2583fps for a 175. This relationship is not true for other powders as shown in the manuals... odd eh? Hence my query.

-=[Bob]=-

[This message has been edited by bald1 (edited August 25, 1999).]

 

[Mal H]

bald1 - Interesting observation about Varget. I hadn't noticed that. I checked the Sierra data for different weight Match Kings. Even the 190 MK needs only 39.6 gr of Varget to get 2400 fps. There must be something about the chemistry of Varget that achieves the same vel. with practically the same amount of powder for different weight bullets. In the words of Artie Johnson, "Velly interesting!".

Other 'traditional' powders (4320, 4895, etc.) require at least 1 additional gr of powder to achieve the same velocity for the 175 MK vs the 168 MK.

[This message has been edited by Mal H (edited August 25, 1999).]

 

[flatlander]

You may find that the difference in bearing surface between the Hornady and Sierra bullets will have more effect on velocity than the few grains difference in weight. The Hornday will generate less pressure because it has less bearing surface, and lower weight, so the higher pressure developed by the Sierra load can more than make up for additional weight.