Question on rate of twist/bullet weight

[MrTom]

I'm about to take the dive on an Accura with a 27" barrel. The rate of twist in this .50 gun is 1:28. Am I going to fare better with the heavier and longer 300 grain fodder or lighter, shorter pills in the 200-250 grain range? I'm getting this with the intent of it being a bh209 gun but will leave the option open to other granular powders. I hunt and hope to work up an accurate load for deer out to about 150 yards max, but in reality most shots will be in the 25-50 yard area. I'll be scoping this Accura with a Nikon Pro Staff 2X7. Many posters have indicated that a 110 grain charge of 209 with 250 to 300 grain bullets seems to be a popular place to start fine tuning a load. I'm just undecided on the bullets weights vs rate of twist.

I've been doing 300 grain .44 cal xtps using 3 Pyrodex pellets in my T/C impact with pretty decent accuracy as a scoped gun but I'm wanting to have this gun for the late bp season where I have to use a low power optic and I am hoping loose triple 7 in either the ff or fff. The pelleted loads I don't think are getting me the accuracy that granular powders are capable of giving, hence the change over to the triple 7 granular. I may be wrong but I think I'm better off in the late season where temps can be widely variable with the t7 instead of the 209.

If there are any thoughts here I'll thank you in advance...

 

[Lee 9]

Well first off the weight does not directly relate to the twist. The relation ship between the length of the bullet and the rate of twist is what controls stability.
The ideal length for a .451 diameter bullet is 1.08 inches.
According to my experience there is very little tolerance on the longer side of that and a bit more on the shorter side.
The velocity enters into the picture, with a 1-28 twist in a 45 for instance a 180 grain shoots best for me with 70 grains of Blackhorn where as the 200 grain will do well with 90 to 120 grains. My recommendation would be for a .451 or .452 bullet of 275 grains and 110 grains of Blackhorn. There are a lot of other combinations that will work but personally I have found having to use the thick petaled sabots with a 44 makes it more difficult to get accuracy and I have had major problems with 777.

where:
• C = 150 (use 180 for muzzle velocities higher than
2,800 f/s)
• D = bullet's diameter in inches
• L = bullet's length in inches
• SG = bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation)
The original value of C was 150, which yields a twist rate in inches per turn, when given the diameter D and the length L of the bullet in inches. This works to velocities of about 2840

For a bullet, the specific formula below uses the bullet's MV and the barrel's twist rate to calculate rotational speed:
• MV(in fps) x (12/twist rate in inches) x 60 = Bullet rpm

For example, a bullet with a muzzle velocity of 3050 ft/s fired from a barrel with a twist rate of 1 in 7-inch (180 mm) (e.g., the M16A2 rifle) spins at ~315,000 rpm.

 

[Fmfdred56]

That twist should shoot all 3 weights ok but the gun will tell you what it likes best.

 

[MrTom]

Thanks guys. Lee....I know what you are saying with the length of the bullets. My Impact will throw a 300 grain .44 caliber xtp tighter than the 300 grain xtp in .452. Same bullet weight but slightly longer in the narrower bullet with the identical powder and charge.

 

[sabinajiles]

FWIW, my Accura V2 and Accura MR shoot several .44 cal bullets, with a Harvester smooth green sabot, very well. The .44 300 gr XTP is more accurate in both than the .45 300 gn XTP. The Cutting Edge Bullets .44 250 gn Maximus is very accurate in both. The .44 265 gn FTX is the most accurate bullet I have ever shot from my Accura V2. It is also extremely accurate in my Accua MR but I really haven't shot it much in that rifle as I settled on the Cutting Edge Bullets Maximus for my hunting load for it because it was extremely accurate and haven't tried many other loads.

 

[tpcollins]

Well first off the weight does not directly relate to the twist. The relation ship between the length of the bullet and the rate of twist is what controls stability.
The ideal length for a .451 diameter bullet is 1.08 inches.

I think weight can sort of affect the twist rate depending on it's composition. There's considerable differences in length between a 250gr jacketed lead bullet versus a longer 250gr Barnes type all copper bullet. Is the 1.08 ideal length jacketed lead or all copper?

 

[Squeeze]

your fine with basically anything you would fit in a sabot. twist is much more important when you get into BIG full bore conicals. You would want a faster twist once you get into something bigger than ~400 grain. maybe even more. The faster the twist = the Longer bullet you can stabilize. The formulas are all about length.

http://www.bergerbullets.com/twist-rate-calculator/

If you understand the math with all the letters and fancy symbols, there are several formulas devised to figure it. Miller formula, greenhill formula, and others. A lot of this was figured over a hundred years ago, and is directly aimed at lead bullets, and their need to be kept at slow speeds (usually under 1400-1500 FPS. faster than that, leading in the bore becomes an issue) So, for long range, more weight had a better BC. Today's jacketed bullets, able to be shot at higher speeds, add another dimension to those calculations. For example, more speed, could stabilize a longer bullet in lieu of faster twist..

 

[MrTom]

Well.....I pulled the pin on the Accura this morning and spent much of the day setting it up. Scoped with a 3X9 now. Bore sighted. I wanted to do a 100 yard check/see in the park behind the house but Ma said "Not today Bucko". She rules so I have to wait to hit the range, maybe Friday. And waiting would likely be easier on the nerves of those out walking on the park trails since we're nearing 50 degrees.

I'm going to enjoy this rifle. As mentioned I plan to use the bh209 in it and plan to start my shooting with 90grain weighed loads using the xtp 250 and 300 grain pills in 45 caliber and the 300 in .44 caliber. I'm going to order some of the 265 grain ftx bullets mentioned and will give them a go later on. Right now I plan to do 5 grain increases and 3 shoot runs of the 3 bullets mentioned. I'll probably stop at 110 grains but if the recoil isn't too bad at 110 I might take it to the 120 grain limit just to see if accuracy gets better or worse or stays about the same. If I have to I'll toy with triple 7 granular in lieu of the 209 but I'm hopeful the 209 works ok for me.

Anyway, I'm excited. I know this isn't a custom gun nor is it up with the Whites and Knights, but its a step up from the impact for me. There isn't much I can say bad about my Optima Pistol and I already like the way the Accura feels in my hands and the way it shoulders. I'm pumped. Thanks for the replies and I'll definitely keep ya'll posted as I learn this new toy.

 

[ronlaughlin]

........ I plan to use the bh209 in it and plan to start my shooting with 90grain weighed loads........

This load is over the maximum recommended by Western Powders. You may be asking for problems with excess pressure, and be putting your self in danger, if you exceed this weight.

 

[MrTom]

You're right Ron. I was looking at my Western Data sheet wrong. I'll be starting at 63.5 grains, weighed. I plan to stop at 74.5 grains, weighed.
I read the wrong line of numbers. It'll be a couple days before I can even think of the range and I plan to pre-weigh charges and tube them. I plan to label the charge weight on each tube. I'm going to do the math and conversion between volume and weight and do 5 grain raises based on weights rounded to the lower whole number after the conversion. I should get a fair idea of the performance of the bullets at 100 yards doing it this way and then the future can be for fine tuning.

Thanks for catching that Ron.