Ogive, General Shape, and Forgiveness In OAL?

Duckford

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Minuteman
Jan 13, 2020
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I was getting back into the reloading bench for my Springfield M21 rifle several weeks ago, took some time off because of work the last few weeks. In any case, I started breaking in the rifle with PMC M80 surplus and eventually some cheapo match loads using the new fired brass and some Speer 168 grain match bullets. Even after I accidentally set the OAL shorter than intended by a casual error, the rifle was shooting 3/4 MOA about 60% of the time in ten round groups (my declining ability and penchant for weak off bipod shooting instead of bench make up most of the 40% remainder), and I was excited for what the rifle might be capable of. Finally got a big box of 175 grain Sierra OTMK and was excited, hoping the bigger bullets would do even better in the 1:10 Douglas. This time I double checked and made sure the OAL was 2.810.

The end result was 2 MOA. I'll admit in hindsight that work fatigue might have played a larger role in the poor performance than I would like to admit, but even when I was calling my shots dead on there was no denying that the accuracy of the cartridge was inferior to the 168 grain Speer. Now, I know that the real answer is to sit down and load a great big batch of bullets with different powder charges and OAL to hunt down the problem properly, the whole incident got me thinking about an issue, especially after asking the good folks at the M14 forum what they thought the problem might be.

Besides the load being a bit too hot for barrel harmonics (the load was perfectly safe for the rifle), they said they would seat the 175 OTMK even further, to the absolute maximum the M14 magazine will allow. Others talked about how they didn't always get good results with high BC heavy bullets. With the bullet shape, this makes sense, and then I thought back to my lead bullet casting and all the literature I read on that subject. That the old Lyman experts talked about general lead bullet design, what worked not only with lead alloys but also what works in loose free bores in rifles and throats of revolvers; that flat nose and big dull round nose are forgiving and generally will work in anything where a spitzer is problematic to nearly impossible to make accurate.

So, after a long winded introduction to the question, is there really a correlation between "dumb" bullets of older and simpler design and forgiveness in throats, that they might be more forgiving with OAL differences? Are modern "smart" bullets of modern and advanced design, meant to maximize BC for a bullet weight, less forgiving with OAL, and potentially more problematic in loose guns? Am I making excuses for bad handloading and shooting, on the wrong track, or is there some truth in this long winded observation?
 

pabrousseau

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Dec 18, 2018
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my experience has been that tangent ogive bullets like you speer and sierra mk and gk bullets are much easier to tune with than secant ogive bullets. I don't know but if I had to guess id say the new sierra tipped bullets are secant ogive. so im saying the answer to your question is yes.
 

clcustom1911

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Oct 23, 2017
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Im very jealous of your rifle. I would love a NM M1A or M21!!

To answer your question about certain types of bullets, yes, some are easier to find the magic combination than others like what @pabrousseau said above. Sierra Matchking, Nosler Custom Comp, and several other brands have a tangent ogive to bearing surface junction and that tends to be a smoother transition from the curved nose to the flat surface which the rifling grabs and twists. A secant ogive is more of a noticeable angle and is finicky with seating depth, but its higher in BC than tangent ogive designs. Gotta give something to get something in the bullet world, generally.

Then we have a relatively newfangled "Hybrid" bullet. That's a bullet that has a short section of easy to tune tangent ogive where the flat bearing surface meets the curved nose, and a sleek secant ogive the rest of the way to the nose. Nosler RDF, Berger, Hornady, and a few others make these. Thay are supposed to be easy to tune for seating depth or "insensitive" to seating depth similar to classic tangent ogive bullets like your older Speer and Matchking bullets.

So, with all that mumbo-jumbo-jibber-jabber said.... it sounds like you have a couple variables going on: powder charge and seating depth. I shot 308 bolt gun for years and only shot the 175 Matchking. While bolt and semi-auto are different beasts, there is a lot of crossover. I'd be willing to bet with your 2.81 load, you're within .020-.030 of the lands.

What's your primer, casing, powder, and charge you use? We can probably help eliminate variables or spot trouble spots with more info.

And by "OTMK" do you mean "Open Tip MatchKing"? Or the Tipped Matchking?
 
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Duckford

Private
Minuteman
Jan 13, 2020
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Im very jealous of your rifle. I would love a NM M1A or M21!!

To answer your question about certain types of bullets, yes, some are easier to find the magic combination than others like what @pabrousseau said above. Sierra Matchking, Nosler Custom Comp, and several other brands have a tangent ogive to bearing surface junction and that tends to be a smoother transition from the curved nose to the flat surface which the rifling grabs and twists. A secant ogive is more of a noticeable angle and is finicky with seating depth, but its higher in BC than tangent ogive designs. Gotta give something to get something in the bullet world, generally.

Then we have a relatively newfangled "Hybrid" bullet. That's a bullet that has a short section of easy to tune tangent ogive where the flat bearing surface meets the curved nose, and a sleek secant ogive the rest of the way to the nose. Nosler RDF, Berger, Hornady, and a few others make these. Thay are supposed to be easy to tune for seating depth or "insensitive" to seating depth similar to classic tangent ogive bullets like your older Speer and Matchking bullets.

So, with all that mumbo-jumbo-jibber-jabber said.... it sounds like you have a couple variables going on: powder charge and seating depth. I shot 308 bolt gun for years and only shot the 175 Matchking. While bolt and semi-auto are different beasts, there is a lot of crossover. I'd be willing to bet with your 2.81 load, you're within .020-.030 of the lands.

What's your primer, casing, powder, and charge you use? We can probably help eliminate variables or spot trouble spots with more info.

And by "OTMK" do you mean "Open Tip MatchKing"? Or the Tipped Matchking?
I'm using PMC M80 brass that I've fired myself from surplus, CCI large rifle primer, 41.5 grains IMR 4895. The powder charge is a bit hot for the brass in terms of service rifle loading, so one thing to do when I get back on the bench is to start reducing charges and see where that takes me with the 175 grainers. I was busy and lazy that day when I loaded the 175 grain Sierras and chose 41.5 grains because it was acceptable and M14 forum guys suggested it.... and my powder thrower and 505 scale where already set for it, so I gave it a try. Same exact combination but with the 168 grain Speer's were giving me the whole 3/4 MOA group potential. Tuning the load down for the heavier bullets is an easy first, and logical, step.

Oh, and OTMK I meant "Open Tip MatchKing".
 
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