I built a nice AR for varmint hunting about 1.5 yr's ago. I put a 24" Shilen 1:8" twist barrel on it so I could shoot the heavy bullets if I ever wanted to. I was always told go with the faster twist and you'll have more options, it'll let you shoot heavier bullets as well as the light ones.

I started out shooting Fed. factory ammo, 55 gr. FMJ's and it was like heaven. I can "ragged hole" 5 shots every time if I do my part at 100 yds. Will hold .5 MOA out to 400 yds. all day long.

When I started reloading for this rifle I was using 60-70 gr. bullets and got the same results.

Changed to a scope with hold over marks and needed to slow my handloads down to match the scope. I got cheap somewhere along the line and deceided to go back to the 55 gr. FMJ's because they were about half the cost when bought in bulk. Tryed to shoot the 55's around 2800-2900 fps and they sucked, this is what brought up my question.

The light bullets shoot great at full power but not so great at the speed of the heavier bullets, 4-500 fps less.

Is there a realationship between weight and speed that makes a twist approate? A fast twist is good for heavier bullets at the slower speed but may be right for a light bullet moving quick enough. Is this what I was told about having more options with the faster twist rate?

You seem to have slow twist & fast twist reversed.

Fast twist has a lower number but higher helix angle. There are optimum twists for bullet weights though with perfect bullets it is not as critical.

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Originally posted by SR4759:
You seem to have slow twist & fast twist reversed.

Fast twist has a lower number but higher helix angle. There are optimum twists for bullet weights though with perfect bullets it is not as critical.

Opps, your right. Fixed it so I don't confuse people.

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The light bullets shoot great at full power but not so great at the speed of the heavier bullets, 4-500 fps less.

Is there a realationship between weight and speed that makes a twist approate?

More appropriately there is a relationship between length of bullet and speed that affects accury..... or has the potential to effect accuracy

Even at 500FPS slower the 55s should be stabilized.....but you say the accuracy sucks. Then step up the speed again and get a different scope.

If one is having trouble with the stabilizing effect of twist he often will see keyholing or oblong holes at the target.....

However a 1-8" will stabilize a 55 grain bullet with ease.....your gun just wants more pressure to make it accurate like it wants to be.....

May I suggest you try a 52 grain hollowpoint boat tail bullet and see how it flys.

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Originally posted by vapodog:

quote:

The light bullets shoot great at full power but not so great at the speed of the heavier bullets, 4-500 fps less.

Is there a realationship between weight and speed that makes a twist approate?

More appropriately there is a relationship between length of bullet and speed that affects accury..... or has the potential to effect accuracy

Even at 500FPS slower the 55s should be stabilized.....but you say the accuracy sucks. Then step up the speed again and get a different scope.

If one is having trouble with the stabilizing effect of twist he often will see keyholing or oblong holes at the target.....

However a 1-8" will stabilize a 55 grain bullet with ease.....your gun just wants more pressure to make it accurate like it wants to be.....

May I suggest you try a 52 grain hollowpoint boat tail bullet and see how it flys.

As soon as this dam snow lets up again I'm going to bump up the velocity and see if my observations are correct. If so, ditch the scope and go back to dialing in my distance.

Never thought about the pressure causing the poor performance, is this more noticable in a semi-auto compared to a bolt action?

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Never thought about the pressure causing the poor performance, is this more noticable in a semi-auto compared to a bolt action?

Personally I think it's simply a "gun" thing.....some like it hot and some like it cold.....I've never owned a center fire rifle in semi-auto.....couldn't say!

Well thanks v-dog. This gives me a few things to try, speed them up and a longer bullet around the same weight. Guess that's what I get for trying to take the cheap route.

I have some 53 gr. Hornady's I haven't tryed through this rifle yet, maybe thats another thing to try.

I guess the ultimate answer is get rid of the scope and go back to what works but where's the fun in that.

The heck with tuning to the scope, make them loads accurate! Try 52 gn Sierra Hollowpoint Boattail Matchkings in that thing with H335 or W748. Also try 69 SMK's with H335, Varget or RL-15 and also 80 gn SMK's with Varget or RL-15. I and many of my friends use the same powder charge of Varget or RL-15 in the 69's and 80's, i.e. 24.5 gns of RL-15 in both 69 and 80 gn lopads, not the same charge as Varget.

If it's a variable power scope in the 2nd focal plane, have you thought about adjusting the power until the reticle DID match the bullet trajectory??? Otherwise, just shoot the sob and find out what yardages the reticle IS on at. Instead of 200, 3, 4, etc... it might be 200, 325, 450, etc...

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Is there a realationship between weight and speed that makes a twist approate?

Yes, there certainly is a relationship between twist, MV, and bullet stability. The faster a bullet travels, the faster it spins as well. Higher RPM makes a bullet more stable. However, in this regard, twist is much more significant than velocity. It is bullet length, not weight, that is the critical factor in determining needed twist rate.