Does a faster twist cause more drift??

[Raptor]

Quick question Guys, I was out shooting this weekend checking zeros on a few of my rifles. I shot my 260 and my 6BRX and 284 from 600yds to 1000yds.

I made sure the zeros were dead on at 100yds then started shooting 3 shot groups at 600yds on out. I had a slight head wind maybe 2 to 3mph and about what I would call an 1/8 value right to left. So if I was going to get any wind drift it should have been to the left.

What I got was groups going to the right about .5moa at 600yd and by the time I was at 1000yds I had 1moa to 1.25moa "Left" wind dialed into the rifles to keep them on center at 1000yds.

I have run into this on one other occasion but never really knew if my rifle was dead nuts on at 100ys. However this time it was. Its usually windy around here so I don't get much chance to shoot without some type of wind adjustments.

SOOooo.... Is this common on the fast twist barrels?? Or what am I seeing with my rifles?? Thanks Guys!!

 

[BoydAllen]

If the CL your bore and the central axis of your scope are not on the same line (as viewed from above) a zero at any given distance may simply represent making the lines converge at that distance, (which is what is going on when you are at the correct elevation setting) from which point they will diverge, and require additional windage adjustment to correct.

 

[Hombre0321]

I don't think ( read that think) that your twist rate is the cause of this happening. Mr. Allen makes a great point about scope and bore and there relation to each other.

Also as you move further in distance the wind is having different affects on the bullet. Another words what happens at the fireing line and what is happening at distances to the target vary greatly sometimes-well most times.

Here is a picture you may well have seen this many times but it is a good thing to review from time to time.

Roland

 

[RDavies]

The drift you are getting is normal for all RH twist barrels. Around 3/4" to 1 1/4" right drift from 100 to 1000yds is normal. If you have a LH twist barrel it drifts the other way.
I have a few barrels which spend 99% of their life shooting at 600+ yards in competition and have solid wind zeros for 800yds, but when I bring them back to 100yds for a crony session, they are shooting 1 moa left.

 

[M14AMU]

The drift you are getting is normal for all RH twist barrels. Around 3/4" to 1 1/4" right drift from 100 to 1000yds is normal. If you have a LH twist barrel it drifts the other way.
I have a few barrels which spend 99% of their life shooting at 600+ yards in competition and have solid wind zeros for 800yds, but when I bring them back to 100yds for a crony session, they are shooting 1 moa left.

+1 Right on!!

 

[mikecr]

What bullet, twist, MV?

 

[RDavies]

I have had the same phenomenon with various calibers raging from 22BRs, 7" twist to 308s with 13" twist, with all the normal long range configurations in between (6mm/8", 6.5m/8.5",7mm/8 or 9" twist). I have a suspicion that the faster twist barrels have drifted more, but nothing scientific or definite.

 

[mikecr]

Gyroscopic Drift Formula from Bryan Litz - Applied Ballistics For Long Range Shooting - 2009 -
(Drift = 1.25 (Sg + 1.2) x (TOF^1.83)

So it's mostly affected by bullet sectional density, twist rate, and air density.

 

[paul223]

Perhaps cant has some influence.

 

[Bert]

I have found that in my 8 twist 6br if i shoot the 80 gr berger fb they spin drift a full min at 500 yrds while the 105 hybrid move almost nothing. I cant get to 1000 with the 80s so i have no info at that range with them. All i am saying is the 80 gr spin drift in the same rifle way more than the 105s. And yes at 1000 the 105 move about 1 min just as everyone else has stated.

 

[Nomad47]

Spin drift (at least in the heavy-for-caliber bullets, which require a faster twist) is ROUGHLY equal to a 1 MPH wind.

 

[4xforfun]

So..let me get this strait.....you have 1/2 moa wind dialed at 600 and 1 1/4 at 1000. So, I don't get it....you have 400 more yards worth of wind to deal with. If you needed 1/2 moa at 600, and there was ZERO wind from 600 - 1000..ie 400 yards of dead air....and you needed 1 1/4 at 1K i would be scratchin my head. BUT my guess is that the wind value was the same the last 400 yards.....you will need to adjust for that 400 extra yards of wind....your bullet is being pushed MORE in the last 400 because the bullet has slowed down considerably. The further out you shoot, the more wind you need to dial.

Let me know if I read your poast wrong, or missunderstood your question.

 

[Raptor]

Thanks for all the replys!! Appreciate the imput. It sounds like the problem is probably the fast twist. This is what I figured but thought I would see what you guys might think.

4Xforfun,
I had a 2 to 3mph head wind about an 1/8 value going right to left. I was asking because this was happening with all three rifles not just one. They are all fast twists, 284 is 1-9, 260 is 1-8 as is the BRX.

Mikecr,
I was shooting the 162 AMAX in the 284 and the 123 Lapua in the 260 and a 107MK in the BRX. I would guess the 284 at around 2950fps+ since it takes 25.5moa at 1000yds. The 260 is 2970+fps (Cronographed) and the BRX would be around 3000fps since it takes about 28moa at 1000yds

 

[mikecr]

And your twists?

 

[Raptor]

Mikecr, the twists are listed right above my reply to you. 284, 1-9, 260 & BRX are both 1-8.

 

[4xforfun]

Thanks for all the replys!! Appreciate the imput. It sounds like the problem is probably the fast twist. This is what I figured but thought I would see what you guys might think.

4Xforfun,
I had a 2 to 3mph head wind about an 1/8 value going right to left. I was asking because this was happening with all three rifles not just one. They are all fast twists, 284 is 1-9, 260 is 1-8 as is the BRX.

Mikecr,
I was shooting the 162 AMAX in the 284 and the 123 Lapua in the 260 and a 107MK in the BRX. I would guess the 284 at around 2950fps+ since it takes 25.5moa at 1000yds. The 260 is 2970+fps (Cronographed) and the BRX would be around 3000fps since it takes about 28moa at 1000yds

AHHH.....I see...I DID missread your post. I got the whole right/left thing screwed up. :-[

1) Are you sure you don't have your scope on with the crosshairs leaning slightly off to the right?...or would that be left?????

I had the same problem last fall. Actually poasted the SAME QUESTION here!!!! 338L with the 300 Bergers. I was CONFERMING my drops out to 1200..something we all should do on hunting guns...throw away those dang computer programs. The further out I got, the more right I hit, even into a very slight wind. I figured it was spin drift or scope cant, but never got back out to the range with that gun before it warmed up to much. I will look into it this fall when the weather turns.

I think that you have been led in the right direction. I know RH twist tubes produce impacts to the right at distance. I never considered that more twist would cause more spin drift.

GOOD TOPIC!!

 

[lrgoodger]

I have found that in my 8 twist 6br if i shoot the 80 gr berger fb they spin drift a full min at 500 yrds while the 105 hybrid move almost nothing. I cant get to 1000 with the 80s so i have no info at that range with them. All i am saying is the 80 gr spin drift in the same rifle way more than the 105s. And yes at 1000 the 105 move about 1 min just as everyone else has stated.

Be careful not to attribute too much to the spin drift difference when comparing the 80 to the 105. You can get a difference in POI (point of impact) between the two bullets because of barrel vibration exit point that is much more significant than the spin drift difference. Shoot a five shot group with each bullet at 200 yards using the same POA (point of aim) and see if they don't group differently even at closer ranges. I have had loads where the barrel exit point tended to send groups to the left and the RH twist spin drift would almost perfectly cancel each other at certain distances.

Even though the 105 A-max and the 105 Berger hybrid are the same weight, I was actually surprised when they shot to almost the exact same POI at all distances. The Berger seems to have a VERY SLIGHT BC advantage since it hit just a tad higher at the longer distances.