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Old March 9, 2016, 10:31 PM   #1
Gregory Gauvin
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Windage - What's going on?

I am a former competitive shooter, on all types of military rifles, but started and ran with a Match AR-15, being vastly familiar. I acquired a PreBan Colt Sporter (1978), to me, appeared to have approximately 20 rounds through it. It's in pristine condition - via bolt face and the apparent lack of wear marks on the bolt carrier, bolt lugs, and brass deflector - I know this gun was barely shot. I had not fired it until today. I wanted to get a 100 yard zero.

I began at 50 yards. I believe it had previously been sighted for 300 meters, as it hit very high. I brought POI to about 1.5" high at 50y and was 1/2" left (if I recall) left windage, with 2 clicks windage (1/2 MOA clicks) was dead center on windage at 50y. I shot a couple of 3 shot strings. Windage was taking out the vertical grid line.

Stepping it out to 100 yards, carbine shot near top of black ring of SR1 Military Target (6 1/4" black circle). I brought up the front post 2 clicks and got my elevation inline with X ring. I was limited on time as the range was preparing to close, however, I am dumbfounded to find the carbine hitting 3-4" left when it was rock solid on at 50y. There was no crosswind - even so, at this range would be negligible... As a trained competitive shooter, I know this windage error being off so much from 50 to 100 was not me.

Is it possible, that when adjusting elevation at 100, the two clicks on the front sight post through off the windage via the post not being truly square?

Secondly...The barrel is a 1:7 twist. I was firing my "plinking" handloads. 25grns of H4895 under a 55 grain. I know 1:7 is best suited for heavier bullets, I found this load to be accurate in my other ARs with 1:8, and 1:9 twists, as well as my mini-14 with a 1:12. No rounds keyholed. This is a Hodgdon minimum load with estimated velocity of 3191, out of a 16" barrel, probably achieving somewheres around 3100. OAL measures 2.240". The twist rate wouldn't have a torquing effect to throw my windage off that much would it?

How could I explain having proper windage at 50 yards and being off by 4" at 100 yards? 1 or 2 clicks I could understand...but 4"?? I'm going to bring the rifle back to 50yards to verify windage there next range session. If I'm off...I know front post is not true. Unless someone else has any thought?
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Old March 9, 2016, 10:39 PM   #2
Sarge
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I have seen front posts (and black powder nipples) so out of true as to be virtually unusable. It has to be the post since that's the only adjustment you made before the windage went all wonky. I would order a half-dozen front posts, best ones you can get, and examine them closely until I found the best one.
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Old March 9, 2016, 11:24 PM   #3
JohnKSa
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Might also be worthwhile to verify that the whole front sight assembly is not canted.
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Old March 11, 2016, 02:22 AM   #4
Gregory Gauvin
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FSB Cant inspection, measurements, MOA Calculations/Conversions

I examined the extension's feed ramp tube to the receiver and found it clocked ever so slightly from center...but I have seen worse, and in those cases, the manufacturer stated they were within tolerance. The extension and ramps were inline...however, I would guesstimate the barrel was torqued clockwise (from shooters position) or counterclockwise (from muzzle end) about 13 thousandths from having the center of the triangle to be dead center between the two feed ramps.

I can't use my flattop receiver to judge whether the FSB is canted since I have a fixed carry handle upper. Via eyeball, it appears to be correct. Granted, this very scientific test I performed, I laid the rifle on the table upside down, both resting on the upper, and I also raised the rear sight as I noted the ear of my right FSB from shooters position did not level on the table. I rotated rifle in several positions to verify my table was level. I used a feeler gauge to measure clearance between ear and table. It measured .004". That's quite a small tolerance - providing the rear sight/upper are within spec.

Ok. Let's say my FSB is can't .004". Using this quick formula:

[Distance to move sight = (sight radius x impact error)/range]

I measured my sight radius to be 14 3/4". [Update - Manual states 14.5"]
100 Yards = 3600 inches
50 Yards = 1800

(14.75 x 4)/3600 = .016"
(14.75 x 4)/1800 = .032"

These numbers are considerably larger than a .004" cant error, but since I was zeroed at 50 yards, and shifted from 50 to 100, we need to multiply that .004" like we would a MOA adjustment. Recalculating, figuring for a .008" sight error:

.008" = (14.75 x 4)/x = ~205 yards. Doesn't make sense to me. The .5" error in sight radius doesn't change the numbers.

I'm going to measure my front sight post with calibers and check for square. Remember, I was zeroed for windage at 50, 180 degree turn resulted in a windage shift.

Front front to back I get .070" - correct per Colt spec.
Side to side I get .070". Post checks out to be square.

However, there is wiggle. More wiggle than I would like. A little side to side play, but much more front to back movement. I am going to compare tomorrow my Stag carbine. How would I get rid of this front post play?? As I gently push on the post the detent nipple moves within the FSB. I am not liking this.

Last check: Every AR-15 I own requires some windage clicks to zero, they all generally shoot left so my windage notch is always to the right. If I remember correctly...my stag carbine requires 7 clicks. Or my match rifle (1/4 click adjustments). Now I can't remember. So I'm going to start counting clicks and making data sheets. I believe my stag 16" carbine has 1/2 click windage/elevation clicks. Irregardless, I THOUGHT my colt sporter would be 1/2 MOA clicks as well, but, I just read the online manual and it stated information for 16" barrel carbines (both fixed and removable carry handles). Rear elevation click values are 1.43MOA...and .65 MOA per click windage. 1 MOA = 1.047 inches, so 1 click elevation = 1.49" @ 100y and 1 click windage = .680". [No information had been given to MOA adjustments per click of front sight post, just 1 click = 1.2cm change @ 25m] Doing the math I calculated 1 click on the front sight post is equal to a 1.728" change of impact @ 100 yards, or 1.65 MOA. If anyone feels like doing math, feel free to correct me. 4 Clicks on the front post.. would equal 6.6 MOA or 6.91". That's pretty close to the standard full 4 click rotation of 5 MOA of a 20" barrel rifle, resulting in a 5.235" POI change. Give or take.

.65 MOA windage clicks is close enough for 1/2 MOA clicks for me. I counted I was 10 clicks windage from center to be on for 50 yards. I strongly doubt I was canting the rifle when shooting at 100. I will have to verify this. However, if I am still 3-4" left at 100, I would need to dial in another 5 clicks. 15 Clicks on the windage seems extreme to me. However, right now with 10 clicks, I am inline with the first hash mark to the right from the center 0. This is approximately where all my ARs zero for me.

Makes me wonder two things. How many MOA does each windage hash mark indicate and taking 10 clicks to move from center to the first hash mark doesn't seem like I have .65MOA clicks, but rather 1/4 clicks. Hmmmm.
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Old March 12, 2016, 01:32 AM   #5
JohnKSa
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I see where you mentioned checking the front post to see if its square, but could it be bent?
Quote:
A little side to side play, but much more front to back movement. I am going to compare tomorrow my Stag carbine. How would I get rid of this front post play??
Front to back play should have virtually zero effect on point of impact. Side to side might be a problem. Maybe try some Teflon tape on the front post threads to tighten things up a little?
Quote:
[No information had been given to MOA adjustments per click of front sight post, just 1 click = 1.2cm change @ 25m] Doing the math I calculated 1 click on the front sight post is equal to a 1.728" change of impact @ 100 yards, or 1.65 MOA. If anyone feels like doing math, feel free to correct me.
Looks good. 1.2cm @ 25 meters is 1.728" @ 100 yards, about 1.65 MOA.
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Old March 12, 2016, 10:40 AM   #6
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Off Axis condition. bore vs sights
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Old March 14, 2016, 01:39 AM   #7
Gregory Gauvin
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I compared my Modern day Stag Carbine to my Colt. Stag makes fantastic rifles. Their tolerances are breath taking. My first rifle was a bushmaster DCM rifle. That rifle is fitted with an accu-wedge. I knew the (former) chief weapons designer at stag personally and was given a tour of the factory. Man...I think they were the first to really produce a top class rifle. Ed Foster, at the time, was the weapons designer who designed and put into production the first left handed AR platform and at the time of my visit was working with AR platform designs in 7.62x39 that took AK magazines. I was with military personal and had clearance into secured areas where their military contract weapons were being worked for special operation units. Handled some of the commando rifles...man they are short teenie little things. Stag Arms, or Continental Machine and Tool at this time was also machine 1911 frames and slides for Colt. Ed was overly enthusiastic showing how tight the frame to slide fit tolerances were. I know other manufacturers have come a long way, but, since Stag were the first to impress me...I suspect they still are the ARs of ARs.

Anyways, comparing my Stag with my colt...my stag carbine has some front sight post front to back play. A shy amount of side to side play...but not measurable in any method then the wiggle one wiggle the other finger test. I will have to say that they are close enough. Like I said, all my ARs zero for windage some clicks right as they all shot left at mechanical zero. I miscounted clicks from mechanical 0 on my colt. I'm dead on at 50 yards with 9 clicks. I counted clicks on my stag and counted 11 or 13 clicks. On my colt, mechanical zero so happens to put the windage knob long O hash at 12 o'clock. With my Stag...with the hash marks lined up with mechanical 0 put the windage knob hash facing the shooter. Hence I say 11-13 clicks. If setting at 12 o'clock, it's 11 clicks. So both rifles are justifiably even. This makes sense...as I never shot my Stag at 50yards...so, if I added to clicks to the colt...I'd imagine I'm going to hitting X marks the spot. I do not recall if the NIB stag upper was given to me set at windage mechanical 0. I do know they factory sight them. All I remember was, I didn't touch the sights and put a soda can at 100 yards and had a spotter watching to call my hits. First round sent took out the can. So I faithfully trust Stags builds...and frankly, I know nearly no AR shoots dead on at mechanical 0 windage...so, there is also probably some human factor involved if 10 clicks to zero is deemed excessive. But more importantly...I'm consistent. Both rifles are .65MOA per click windage.

My Bushmaster DCM takes 7 clicks to for zero. Albeit...that rifle has 1/4 clicks. And a 20" barrel. .65MOA I presume are 1/2 clicks. 1/4 clicks would be...I dunno exactly the MOA number, but roughly 1 1/2" at 100. There's also the fact that I've shot so much with a Match width front sight post that these carbine posts appear as thick as a brick me to.
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