5.56mm Ballistics, can you help me with ACOG issues?

[Charlotte the flyer]

sign85

I'm hoping that the collective minds that make up Full Bore can be me some advice / explanation regarding ballistics, barrel lengths and two ACOG scopes that I've got. If anyone doesn't know what an ACOG is, it's a 4x scope (although some are 6x) with a etched bullet drop compensator built into the optic.

http://savannaharsenal.files.wordpress. ... 2-full.jpg

I have a TA01 which is currently mounted to a carry handle topped M4 carbine (it came with the rifle). At close distances it's OK against the etched reticle (<300mards) but further out I need to add a bit on to the elevation. I've checked the Trijicon website, this scope is designed for use on top of the carry handle of an M16A4 with a 20in barrel using a 55gn round. It's close enough for me and I know that I just need to add a little more when shooting further out.

I also have a TA01NSN which I'm hoping to use on the L98 (it's genuine but I got it off Ebay model unknown). Trijicon say that this is for use on a flat top M4 with a 14.5 in barrel with a 62gn round. I'm a little more cautious with this one because I'm guessing that it's going to shoot higher but I don't' know how much. I'll be using GGG which I believe is 62gn.

After this ramble my question is : which is the most important factor in bullet drop and comparison to a reticle. Is it the height of the scope above the barrel, the barrel length or the weight of the round. The round will be the same as spec (62gn) so will the height above the barrel (flat top), it's just the barrel length will be longer so presumably the round will have more fps. If it's going to be a couple of inches out I'll stick it on, if it's feet then I think I'll go back to the SUSAT.

[SevenSixTwo]

There's a lot of confusion over BDC. I use a TA31F and it's not too badly out at 100 and 200m but I've not shot it at 300m+ yet.

My SpectreDR 1.5-6 on my .308, however, is a different kettle of fish:

POI = POA at 100m with the 100m reticle (zeroed).
POI = POA -2" or so at 200m with the 100m reticle.
POI = POA at 300m with the 200m reticle.

I suspect in this case the anomally is caused by the rifle being straight-pull and all the gases being used to propel the round forward so, naturally, the BDC will be out of kilter with what it's designed for - semi auto rifles with a lower MV. I dunno - just a hunch - I'm no ballistics expert.

Btw, I use 62gr and 147gr with the ACOG and Spectre respectively - again; what they're designed for.

BDC isn't meant to be a precision system - just about getting fist-sized groups in a chest-sized target at a given distance (but it's the way I'm happy to roll). In the end, I guess it's just a case of getting to know your scope plus what gr bullets you're using and staying consistent.

Re: height over bore, I'll not hazard a guess but can you not mount it on the receiver?

[Charlotte the flyer]

Re: height over bore, I'll not hazard a guess but can you not mount it on the receiver?

Oops, sorry I should have said, yes it's mounted onto an LEI Picatinny so the height would be roughly the same, the cartridge would be the same but the barrel length is different.

[SevenSixTwo]

Barrel length will definitely affect BDC results. I'd say consult the Trijicon site but I've even found that contradictory in some places. Try searching/joining the forums at M4Carbine.net too - much better placed to advise on the ACOG/AR-15 platform than here.

[Charlotte the flyer]

Thanks, I'll give that a go. I've tried looking down both optics to compare them. At 15 yds (or one back garden length) the 500m mark on the TA01 is the same as the 400m mark on the TA01NSN

[Blackstuff]

You must have supermodel cheek bones if you can get a cheek weld using an ACOG on top of a carry handle!

Tbh I think you're over thinking this, have you actually tried shooting the rifle/scope using the BDC and NOT hit the target? I've used my ACOG on my (now sold ) Steyr AUG, a platform it certainly wasn't designed for, and got consistent hits out to 600m on a military electronic targets range (I.e torso sized targets). You aren't going to get proper precision out of any military BDC, they're just there to improve squaddie accuracy

[Charlotte the flyer]

Think I might just put it on and give it a go then.

It's on an LEI rail straight onto the top of the receiver! I'm more Super Tramp than Super Model

[saddler]

Velocity is the thing, along with correct bullet weight

[leeroy7031]

sign85

I'm hoping that the collective minds that make up Full Bore can be me some advice / explanation regarding ballistics, barrel lengths and two ACOG scopes that I've got. If anyone doesn't know what an ACOG is, it's a 4x scope (although some are 6x) with a etched bullet drop compensator built into the optic.

http://savannaharsenal.files.wordpress. ... 2-full.jpg

I have a TA01 which is currently mounted to a carry handle topped M4 carbine (it came with the rifle). At close distances it's OK against the etched reticle (<300mards) but further out I need to add a bit on to the elevation. I've checked the Trijicon website, this scope is designed for use on top of the carry handle of an M16A4 with a 20in barrel using a 55gn round. It's close enough for me and I know that I just need to add a little more when shooting further out.

I also have a TA01NSN which I'm hoping to use on the L98 (it's genuine but I got it off Ebay model unknown). Trijicon say that this is for use on a flat top M4 with a 14.5 in barrel with a 62gn round. I'm a little more cautious with this one because I'm guessing that it's going to shoot higher but I don't' know how much. I'll be using GGG which I believe is 62gn.

After this ramble my question is : which is the most important factor in bullet drop and comparison to a reticle. Is it the height of the scope above the barrel, the barrel length or the weight of the round. The round will be the same as spec (62gn) so will the height above the barrel (flat top), it's just the barrel length will be longer so presumably the round will have more fps. If it's going to be a couple of inches out I'll stick it on, if it's feet then I think I'll go back to the SUSAT.

The details I read on the trijicon site when I purchased my ta01 for a14.5" stated that at 600m the difference in bullet drop would be equal to the thickness of the stata line, translating to it makes no difference in using the ta01 on a carbine. I believe this is also true to a ta01nsa to using a 20". IIRC the only difference was that these didn't have the additional graduation window within the reticle out to 700 & 800m and they are compatible with NV.

At kingsbury did you need to aim off at 600m ?

[Maggot]

All I have to offer is that in my days of hunting with an air gun I simply worked out by practical testing the POA at various ranges, it soon becomes instinctive (or at least as good as your rangefinding becomes).

I also knew my point blank range and primary and secondary zeros, you have to in order to make sure you give bugs a good send off with something looping like a tennis ball!!

Having only ever used battle optics (SUSAT/SUIT/IWS) and irons on service rifles, it was always a case of "Aim here to hit here at this range in this wind".

I am starting out in CSR with a No4 but have an Elcan Spectre for 5.56. It should come in reasonably well as it will be fitted in the same way and to a similar rifle as it was designed for, but I wont have a ready supply of the parent ammo so it will be a learning curve too I am afraid. I will have to put the work in to get any returns.

Funny, I was sure the markings on the old Fig11s were rectangular for a reason....could it be allwoing for slight range errors with spikey rets or irons not making much of a dfifference but still signifying the fact that the round would have done its job

I wont mention the w word, but they are battle rifles and as such a certain ammount of refinement has been taken away for cost, robustness and simplicity...then its down to familiarity and the skills of the user....oh heck!!

Imagine the fun when PFC Audy Murphy forgets to zero the windage or put the elevation drum back to secondary zero.