5.56 Ballistics of 55gr vs 62gr. vs 77gr.

[Baldheaded]

Question regarding an AR15 zeroed with 55gr. ammo and considering changing to 62gr. or 77gr. ammo. Should you stay with a certain grain ammo like 55 once you get zeroed or can you change to 62 or 77 without getting off zero by to much? I could probably figure this out myself at the range by trial but I don't get out much this time of year. From what I have been reading it appears 62gr. ammo is a better all-round go to. I also read that when going out at distance a 77gr. bullet is better. I am sure this has been touched on here before somewhere but I still would like to hear from you guys about this. I guess it can't hurt to just have some of all the ammo and use it based off your needs. Does going from 55 to 77 really throw off the zero by that much?

 

[TheOriginalMexicanBob]

It depends on your barrel’s rate of twist.

 

[Baldheaded]

It depends on your barrel’s rate of twist.

16.5" 1in7 twist.

 

[TheOriginalMexicanBob]

A 1:7 twist favors heavier bullets.

 

[smokey]

Just download hornady's ballistics program and punch the loads in.

In general, no, you will not have different loads hit the same place. This is especially true with different grain weights. You'll need to zero your gun to a specific loading or make adjustments if switching to another loading. Obviously the further out you get, the more variation they'll be.

It won't differ just up and down either. Different loads may print anywhere up, down, left, right, or a combination of two. They'll also print different sized groups, depending on which load your barrel likes the best.

 

[Defense Initiative]

Question regarding an AR15 zeroed with 55gr. ammo and considering changing to 62gr. or 77gr. ammo. Should you stay with a certain grain ammo like 55 once you get zeroed or can you change to 62 or 77 without getting off zero by to much? I could probably figure this out myself at the range by trial but I don't get out much this time of year. From what I have been reading it appears 62gr. ammo is a better all-round go to. I also read that when going out at distance a 77gr. bullet is better. I am sure this has been touched on here before somewhere but I still would like to hear from you guys about this. I guess it can't hurt to just have some of all the ammo and use it based off your needs. Does going from 55 to 77 really throw off the zero by that much?

 

[brianns]

With heavier bullets the velocity may be slower if loaded with less powder. So the overall trajectory could change. Also consider if the 62g bullets have a penetrator in them. Folks say those projectiles aren't as consistent with their construction and may not provide as much accuracy as the lead filled FMJ rounds. So decide which bullets you want to depend on and then zero for those but don't expect the same performance from the others.

Worse case you can record how they do with the one zero. Like x bullet drops sooner at y range than the bullet the rifle is zeroed with. Or notes such as 62g XM855 gives me 2.5 MOA but 1.0 MOA with 77g. You can use ballistic calculators to predict it all but then check with actual shooting.

 

[Biggfoot44]

Still not quite what the OP is asking.

I think he's looking for :

Zero for your prefered loading .
Then experimentally see where other bullet weights impact in relation , and with luck , at least one would have a potentially useful POI .

 

[RCH]

Good guidance from the group. Depending on what role you perceive for your rifle and the distance you plan on shooting out to, the different variables in the various bullet weights and velocity will have an effect on the bullet drop the farther you go out. Federal has a nice online ballistics calculator that may be a good starting point for you to develop a spreadsheet on drop and wind/temperature effect for your particular rifle:

Sites-VistaFederal-Site

Federal Ammunition

www.federalpremium.com

Other ammo manufacturers may also. This could be a fun little project to occupy your time on over the winter, and then fine tune it when you get to the range.

 

[outrider58]

All things being equal, you won't really start seeing a huge(really not that huge) difference until to get into the mid to upper 70s.

 

[KRC]

My 55 gr. SBK, 69gr. SMK and 77 gr. SMK loads each have different 100 yard zero's and downrange ballistics (the 69's and 77's are close at 100 yards). These values can easily be determined at a range and the differences recorded and compensated for (adjust elevation and windage) when switching ammo. A ballistics program like Strelok/StrelokPro is also useful in this regard, as is the MV for each load.

 

[Ponder_MD]

View attachment 445920

Oooooooh, neat graph! I have several weights to try out in my stash.
I've already observed that 62gr is better than 55gr in my rifle but I haven't tried anything heavier yet.

 

[Park ranger]

at 100 yards you could load a mag with all three and not notice a huge point of impact change. Go try it and see for yourself.

 

[Melnic]

If you are buying factory ammo, just changing brands can affect Point of Impact (zero) even if you don't change bullet weight. And bigger effect the further you go out. Depends on what your doing. Shooting at targets from bags vs shooting big steel plates offhand (may not even notice)

 

[outrider58]

at 100 yards you could load a mag with all three and not notice a huge point of impact change. Go try it and see for yourself.

This

 

[Uncle Duke]

If you are buying factory ammo, just changing brands can affect Point of Impact (zero) even if you don't change bullet weight. And bigger effect the further you go out. Depends on what your doing. Shooting at targets from bags vs shooting big steel plates offhand (may not even notice)

Yep! Very true of handloads as well.

There is of course, such a thing as "good enough" for each application. Needs differ when shooting large sized steel gongs, or small prairie dogs, or for exacting precision on paper. But yes, different loads will most typically have some degree of point of impact variation. Question becomes by way of application, whether or not it matters to the guy behind the stock.

 

[spoon059]

at 100 yards you could load a mag with all three and not notice a huge point of impact change. Go try it and see for yourself.

Yup. Depends on your zero and distances shooting, but it's likely a small shift.

 

[redsandman6]

i would suggest using a ballistic calculator. punch in the data and it will give you how much each bullets drop, then you can create a dope chart. Ballistic Coefficients and velocity all matter. the farther the distance the more is matters.

i shooter and StrelokPro shooter gives a nice chart that has bullet drop in inches as well as mil/moa

he is a test out to 300 yards

 

[Doco Overboard]

You have your rifle zeroed with a particular weight of bullet choice.
Because a heavier bullet may have less velocity, doesn't mean its going to have a point of impact thats lower, it could have a mean point of impact thats higher.
Because a heavier bullet makes more pressure. And, a heavier bullet choice wont shed its velocity as quickly as a lighter one.
Being that your rifle is zeroed, shoot some of your new ammo for a constructive zero and determine what happens.
The other thing is,a heavier bullet makes more recoil. So the muzzle is moving upwards before rate of twist or anything like that has a more profound effect.
The bullet hasent even left the barrel yet when recoil begins.

 

[alucard0822]

The big question, why do you want to change ammo? what are you using it for?

First off, ANY ammo change can change the point of impact, velocity, bullet shape, of course weight, even conditions like wind and temperature can change the zero on any given day. Zeroing will be dependent on many factors as well, to hit POA at 25 yards, for a 0-300 "battle zero", or 100yd zero + the scope mount MOA to use as a base for adjustments with a precision rifle dope card.

The difference in point of impact might not matter, or it can matter a lot depending on the application. A 3MOA shift going from 55gr FMJ cheap range ammo to 77gr MK262 won't matter if the purpose of the change is for a better home defense round. Might be a huge difference if you are swapping for a precision rifle at a 600yard range. IMO 62gr SS109/M855 is pointless unless it was issued to you. 55gr FMJ tends to outshoot it in most applications being the steel core in 62gr milsurp ammo isn't usually concentric, and just enhanced hard cover penetration a little, till better ammo like mk262 came around. Of course there are some 62gr FMJ without a steel penetrator or other bullet designs that are fine, but not likely to have a big improvement over 55gr in accuracy or BC at range, they may be better for hunting or other uses.


Keep in mind, the twist charts are more guidelines than anything, and some things like a boat tailed FMJ with a little longer bullet than a flat base can make one 55gr shoot better than another 55gr, then there is barrel length and velocity, shorter barrels can use a faster twist to get the same stabilizing RPM as a longer barrel/higher velocity.

If the reason for the 55s is just general use at a range 200yds or less, then no reason to spend more on different ammo IMO. If the reason is defense, can still practice with 55s, but fire enough 77gr MK262(my recommendation for defensive use) to ensure reliability, and at least to be aware of where it hits at closer ranges, being the line of sight over the bore will be more important at close range than minor differences in impact at longer ranges. If it's for precision, and you plan on shooting past 200yds, then try a few fed 69gr and 77gr Fed GMM or handloads, and see what it likes best. Heavier will have a higher BC to fight wind and velocity loss at longer ranges, but even then some rifles will group much better with a bullet that might be less ideal in ballistic charts.