sectional density and long range bullets

[olympian]

Does anyone know why the target bullets available for 6.5mm at the top end of the spectrum have a significantly lower sectional density (SD) than 7mm and .308?

Sierra offers the 142gr MatchKings (MK) with a SD of 0.291, while the 7mm and .308 bullets offered have SDs as high as 0.310 for the 175gr. and 0.301 for the 190gr., respectively. Berger offers even heavier 7mm bullets (0.319) and there are several .308s in the 200gr. range. With this in mind, does anyone know why the 6.5mms do not have low drag bullets in the 150-153gr range, which would put the SD in a similar over 0.300 range? There are plenty of cartridges using 6.5mm bullets theoretically capable of pushing them to 2600-2700fps. in modern rifles.

This would be a niche market but the Sierra 7mm 175gr. MK & Berger 7mm 180gr. VLDs are quite impressive. Thoughts?

 

[Steve Blair]

Sierra makes (made?) a 155 grain 6.5 MK with a sectional density of .318. I do not see it on their web site but was formerly catalog number 9570. Attached is the PDF of all Sierra bullets.

 

[olympian]

The old Sierra 155gr Palma bullet has not been made in a few years and it has a lower ballistic coefficient (BC) than the 142gr. It has a similar profile/shape as the 140gr Match King, which is not as good of a long range target bullet as the 142gr. Match King or Lapua 139gr. Scenar. I believe I once heard someone say, "It is old technology."

Sierra and Hornady make 160gr. 6.5mms, but they are hardly target bullets. Why are heavier VLDs, and thus BCs with proper design, not readily available?

 

[Laurie]

I'm a little puzzled as to why you're bothering about SD in isolation in target bullets. It's BC that counts (allied to MV) and that's a combination of SD and bullet shape. The larger calibre bullets at the heavier end of their usable weight range will always have higher SD values - you can make a usable 300 or 350gn 0.338 bullet, but 90gn is pushing 0.224" and that limits the SD.

In practical terms, optimised 7mm bullets (175-180gn) will always have a higher BC than their equivalent 6.5s, but that relationship applies up and down the calibre range too, so 6.5s outperform 6s, while 7s are inferior to optimised 30s. If SD/BC were all that mattered, we'd only shoot .338 Lapua Magnum or .50BMG, but there are other factors too like rifle weight and recoil to be taken into account. For a long time, it was believed that a mid-size 6.5 like the 6.5-284 with an efficient shape of 140-142gn bullet was the optimum long-range match rifle mix. Now, a lot of people reckon that 7mm with 175-180gn bullets in something within the .284Win to 7mmWSM case size range is the optimum. It's certainly unbeatable in F-Class in the UK, and no top shooters use a 6.5 anymore.

The underlying question you're asking is why aren't there lots of 155gn 6.5mm match bullets on the market. I imagine that Lapua, Berger, Hornady, and Sierra have come to the conclusion that the optimum weight for long range shooting is 139-142gn. After all, the Swedes and Norwegians started off with almost the heaviest bullet you can practically have in this calibre, 156gn and that in round-nose form, in the 6.5X55mm. They later switched to a 139gn FMJ spitzer after a lot of experimentation because that weight and shape combination gave optimum external ballistics. The laws of ballistics haven't changed since even if we do have much better powders and components.

The Scandinavian ammo companies long produced 155-160gn soft-point bullet ammo, and probably still do, but that's for use on tough deer species such as moose and caribou where you want maximum SD for penetration and punching through big bones, and long-range ballistics don't matter in that form of hunting. (Yes, the Scandinavians use the 6.5X55mm on 1,000 lb plus animals!)

Laurie,
York, England