139 vs 140 gr 7mm bullets

S

stillquietvoice

New member
i have checked several sources and found what seems to be a large difference in powder charge and coal between 139 and 140 gr bullet weights.
my question is twofold. first why is the oal shorter for the heavier wt 2.750 vs 2.800 and second why reduce charge wt wt 1.5 gr for only a one gr increase in bullet wt.
the bullets im useing are horn 139 sst and sgk 140 spbt. in front of imr 4350.
 
The bearing surface of the 139 is much shorter than the SGK, which is a Boat Tail. I've never gotten good accuracy in my 7mag or 280 from the 139. I liked 145-160, particularly the 154 Hornady.
 
both sst and sgk are boat tails with similar ogives. im shooting 7-08 rem. the 140 just seem to be the odd weight out for oal. in my rem 700 both produce one hole groups, slightly different poi though not more than an inch apart.
 
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While they were writing the load manual the Ouija board wiggled.
The same type of inconsistencies can be found in Astrology.
Mine says today is a good day to post on a reloading forum.
 
>first why is the oal shorter for the heavier wt 2.750 vs 2.800 and second why reduce charge wt wt 1.5 gr for only a one gr increase in bullet wt.

Are the bullets the same in construction? Both jacketed and lead core? Both plain base or both boat-tail?
Next, Max pressure is reached at different charge weights even with the same weight bullets of the same construction due to other variable.
Variables that immediately come to mind that effect pressure are:
A) the exact bullet geometry (you'll be surprised how much variation there is in meplat and ogive and bearing surface for bullets of the same weight from different manufacturers that look identical to the eye)
B) the exact lot of powder used (powder does vary lot-to-lot and sometimes by quite a bit, such that every new lot of powder really needs to be treated as almost a completely new load work-up—so, if you find a lot of powder that works well, stock up on it)
C) the COL
D) the cases used (different internal volume)
E) the primers used
and
F) the chamber of the gun used.
Thus, one manual will differ from another and both will differ from your results as none of them used all the same components (and you know none of them used the same gun, much less used YOUR gun).
We are lucky that things are a consistent as they are.

COL in a rifle is determined by:
1) the length of the magazine (the cartridges have to fit the magazine and not bind)
2) in some case with rifles, the feed system/geometry (lever action)
3) the length of throat (where the ogive of a particular bullet hits the lede/rifling (as mentioned above, this varies quite a bit for different bullets)
and
4) if you use it, the location of the cannelure.

In general, I ignore the COL listed in a manual, as it has almost no bearing on my rifle and my bullet. I think it was better when manuals didn't list COL so no one thought the COL listed actually was important for their rifle.
First, I use the longest COL that fits the magazine (internal or external) without binding on the magazine walls and chambers without problem. So far, that has always worked.
I have not found a need to crimp in 40 years, but some do, so the COL in that case must agree with the cannelure.
Once I know I have a COL that works in MY rifle, I start load work up.
 
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The difference is because you are comparing different manufacturers, it's not the bullet weight. Some bullet designs have more surface in contact with the barrel increasing pressure even though the weight is exactly the same. Those bullets need less powder to achieve the same pressure. The hardness of the bullets jacket can change things as well.

If bullets from different manufacturers have very similar shapes and the jackets are similar the load data is the same. In fact I can often use exactly the same load data to load 150 and 155 gr bullets in my 308 depending on the exact bullet manufacturer. But some 150 gr bullets use different load data than other 150 gr bullets from other companies.
 
The bullets Hodgdon tested was 139gr Hornady SP and Sierra 140gr SP

Only different is using 1.5gr more powder for the Hornady less pressure and 1.5gr less powder with Sierra but more pressure which amounts to gain of 33fps max loads.

If you used different powder like H-4350 you would use 2gr more powder, less pressure and a gain of 40fps with Hornady bullet.

Most of the powders list the gain in velocity/less pressure is with 139gr Hornady and some could be 100fps difference.
 
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