Lighter bullets hit to the right and string out - why?

[FocusGroup]

I'm hoping some of the high precision target knowlege in this area of the forum here might help. I have a Browning x-bolt in .243 and I notice that once i move lighter than 95 gr the bullets start to string out to the left and high. I expect a lighter bullet to hit higher, but I don't understand the stringing out to the right. Furthermore, it's not even a consistent amout to the right - instead, they string out diagonally. With certain 95 grain bullets that rifle will shot well under an MOA. However, with 55 grain bullets it was spraying them over about 10 or 15 inches diagnonally at 200 yards. It's a 1:10 twist barrel, factory bedded.

It's almost like the lighter bullets are swerving or something. Any explanations of the physics that are at play here or now to get light bullets to behave?

 

[6ShotsOr5?]

Before we attempt any answers, let’s clarify what you did. Are you saying you just kept the same powder charge but switched from a 95 grain bullet to a 55 grain? Or, are you saying you repeated your whole load development process and you couldn’t find a good load for the lighter bullet?

 

[Warren Dean]

2 words....barrel harmonics.

 

[FocusGroup]

No reloading involved. Just changed from Winchester 95 grain white box CT bullets (I think that's what they're called to same thing with 55 grain. Also noticed other lighter bullets such as 70 grain start to string out to the right but it's more pronounced with lighter bullets. O.k. what about harmonics would cause the stringing?

 

[FocusGroup]

I guess I understand some loads shooting tighter groups than others, I don't understand the tendency for lighter bullets to string out diagonally.

 

[FocusGroup]

Actually, the 95 grain bullets shoot pretty well at less than 1 MOA out of the stock x-bolt 1:10 barrell. They don't seem to make pre-loaded .243 much heavier than 100 grain. I thought a slower twist was better for lighter shorter for caliber bullets - faster for heavier?

 

[NZ_Fclass]

2 words....barrel harmonics.

another word - OAL and jump, OK thats a three letter acronym and a word with a conjunction.

 

[Ned Ludd]

Sub-optimal barrel harmonics are unlikely to be the main cause of 10-15 inch groups at 200 yd. I'd be immediately checking for something coming loose if I ever saw groups that large at such a relatively short distance (i.e. scope mounts, action screws, etc.). Nor is it likely that the barrel twist rate is the primary culprit, as the issue apparently grew worse when the bullet weight dropped below 95 gr, which typically also means as the bullets became shorter. If it was mainly a twist rate issue, precision would become worse as the bullets lengthened.

From the limited information provided, I can't tell you with certainty exactly what IS happening, but first I'd check to be certain all screws on the rifle/scope are tightened appropriately. Then I'd carefully check the action bedding and barrel channel to be absolutely certain the barrel isn't contacting the stock and/or the bedding doesn't have any obvious flaws. After that, I'd try shooting a few more groups with both the ammo that grouped acceptably well (95 gr), and the types that shot poorly, except I'd only shoot them at 100 yd distance, and at a time with minimal (preferably zero) wind. Take pictures of the targets, so you can post them here to facilitate and elicit further analyses and suggestions. Velocity data would be nice if you can, but not collected with a chronograph that touches the barrel (i.e. MagnetoSpeed). Anything you can do to better illustrate and quantify the issue will only help with the diagnosis.

 

[Jennb]

If I read correctly this is purchased box loaded ammo? It would be a great improvement to reload your own ammo.

 

[FocusGroup]

Sub-optimal barrel harmonics are unlikely to be the main cause of 10-15 inch groups at 200 yd. I'd be immediately checking for something coming loose if I ever saw groups that large at such a relatively short distance (i.e. scope mounts, action screws, etc.). Nor is it likely that the barrel twist rate is the primary culprit, as the issue apparently grew worse when the bullet weight dropped below 95 gr, which typically also means as the bullets became shorter. If it was mainly a twist rate issue, precision would become worse as the bullets lengthened.

From the limited information provided, I can't tell you with certainty exactly what IS happening, but first I'd check to be certain all screws on the rifle/scope are tightened appropriately. Then I'd carefully check the action bedding and barrel channel to be absolutely certain the barrel isn't contacting the stock and/or the bedding doesn't have any obvious flaws. After that, I'd try shooting a few more groups with both the ammo that grouped acceptably well (95 gr), and the types that shot poorly, except I'd only shoot them at 100 yd distance, and at a time with minimal (preferably zero) wind. Take pictures of the targets, so you can post them here to facilitate and elicit further analyses and suggestions. Velocity data would be nice if you can, but not collected with a chronograph that touches the barrel (i.e. MagnetoSpeed). Anything you can do to better illustrate and quantify the issue will only help with the diagnosis.

o.k. good idea. I'll shoot some groups and take pictures of the box and post once I get back to that rifle. I left it in another state for the winter. I'm pretty sure everything is tight on it but will check again. This phenomenon seems related to bullet weight as I notice the stringing to the right starting to happen with 70 grain of various makes and get more extreme with 55 grain. Once I'm shooting heavier 90 grain plus some bullets shoot better than others but in a more "normal" just random pattern. The Remintonn core lokt (green and yellow box - I think those were 100 gr) shoot pretty bad like 3" groups, the Winchester white box silvertip CT shoot very well - under an inch, federal yellow box 95 grain shoot very well also. Based on that I don't think anything is loose unless it's the stock touching in such a way it only affects the lighter bullets. You hear abut 'overstabizeing" -- I wondered if the 1:10 twist spun the lighter bullets fast enough at a higher muzzle velocity that the spin caused them to swerve or something.

 

[Link]

Is this a typing error?

However, with 55 grain bullets it was spraying them over about 10 or 15 inches diagnonally at 200 yards. It's a 1:10 twist barrel, factory bedded.

 

[Ned Ludd]

o.k. good idea. I'll shoot some groups and take pictures of the box and post once I get back to that rifle. I left it in another state for the winter. I'm pretty sure everything is tight on it but will check again. This phenomenon seems related to bullet weight as I notice the stringing to the right starting to happen with 70 grain of various makes and get more extreme with 55 grain. Once I'm shooting heavier 90 grain plus some bullets shoot better than others but in a more "normal" just random pattern. The Remintonn core lokt (green and yellow box - I think those were 100 gr) shoot pretty bad like 3" groups, the Winchester white box silvertip CT shoot very well - under an inch, federal yellow box 95 grain shoot very well also. Based on that I don't think anything is loose unless it's the stock touching in such a way it only affects the lighter bullets. You hear abut 'overstabizeing" -- I wondered if the 1:10 twist spun the lighter bullets fast enough at a higher muzzle velocity that the spin caused them to swerve or something.

In general, it is better to slightly overspin a lighter/shorter bullet than to underspin a longer/heavier bullet. Certainly there is likely to be a limit to how much you can overspin a bullet without noticeably decreasing precision, even though aerodynamic jump will increase. The true limit that comes to mind is when a bullet is overspun to the point of jacket failure, which is pretty obvious . Nonetheless, running the shorter/lighter bullets in a barrel with a slightly faster-than-necessary twist rate should not result in 10"-15" groups at 200 yd; i.e. bullets usually don't "swerve" nearly that much unless something major is wrong. As NZ_Fclass mentioned, the shorter/lighter bullet loads may well be jumping farther in your chamber. If that is something they don't "like", poor precision could easily be the result. If you haven't already, you can easily measure CBTO for a few loaded rounds of each to make an estimate of how different the seating depth is with the different bullets/loaded rounds.

 

[LCazador]

I'm going to bet a dime to a donut that the problem is two different ogive bullets. The 55's and the CoreLokt are both secant ogive bullets. The rest are probably tangent ogive. That freebore like most rifles of today probably has a huge jump and the heavier bullets with a tangent ogive are going to shoot better. A 10 twist is not going to over spin even a very short bullet like a 55 grain. I just recently shot 55's out of my 8 twist 6BR just to burn up a bunch of 55's to fireform brass. They shot very well but I don't have that factory freebore! My expert advice stay from the ammo that doesn't shoot well. Stop trying to second guess your rifle. It is what it is.

 

[dryhumor]

If you are shooting factory ammo in most instances you’ll have that…. You seem to be shooting factory ammo so I’ll carry on.

Not every ammo type shoots the same. There are different powders with different burn rates, different primers with different heat rates, different bullets with different shapes, and loaded cartridges with different lengths and bullets seated at different points.

All of these items combine to have a unique effect on the barrel steel as the pressure wave creates a harmonic vibration at ignition. The instance the bullet leaves the barrel the muzzle must be as still as possible. When it is as still as possible accuracy is possible while discounting all the other variables.

As an example I once had a 308 that shot factory Federal Gold Medal match ammo at 1/2 MOA most days. After the barrel was threaded, it did not. Removing the steel to thread the barrel allowed the muzzle to move as the bullet exited, whereas before it was very still.

I ended up reloading for that rifle to get back to 1/2 MOA.

 

[Old Navy]

Just a wild guess but am thinking a mile of free bore. Bought a well used XP-100 in 7 BR with a well worn throat. My favorite load for my other XP was with 140s. First group out of the barrel was something around 6" with one key hole. The gun came with a few rounds of 168s if memory serves me right. Don't remember what the group was but it was a lot better. Ended up getting a new take off barrel from the previous owner.