Hornady oal gauge and compound throat

[Rbartho]

First post here, so first let me say thanks to all you regulars for the great information on these boards.

I have an AA 18 inch upper with the SAAMI chamber. I used the Hornady oal gauge with the 123 sst and the bullet stops at an oal of around 2.245. That seems to be much shorter than what many of you list as your suggested oal in the Reloading Handbook VOL II.

With the design of the compound throat, is it possible that the bullet is touching the throat instead of the lands? Or more likely I just have a shortish chamber?

Thanks,
Ross

[Drillboss]

Touching the throat is the same thing as touching the lands. Here's a diagram I put together of the SAAMI chamber dimensions. Note that the vertical axis is showing radius rather than diameter.

Chamber Diagram.pdf

My 16" AA barrel measures 1.68" for a 123 gr SST using the Hornady 26 comparator for cartridge base to ogive. That works out to a COAL of about 2.26". Assuming our comparators are the same, your chamber does seem to be a little short.

Call AA and tell them what you're seeing. If there is a problem, I bet they make it right.

[LRRPF52]

2.245" is the factory COL for the 123gr SST.

The thing with this throat is distinguishing between where the initial .5 degree leade begins, and where the final 1.5 degree is.

The shallow initial leade is very forgiving for engagement in most cases, and will extract normally with charge handle retraction.

If you look in Volume II, there are two samples that used 2.230" COL.

Make a dummy cartridge with the 123gr SST in sized brass. Load it to 2.245" since that's where it appears to stop for you right now.

Color the ogive completely with black permanent marker.

Carefully chamber the dummy cartridge, making sure it doesn't touch the feed ramps, with a full seat into battery.

Carefully extract it, making sure it doesn't get shoved by the ejector into the barrel extension teeth by using your finger in the ejection port to stop it.

Look at the projectile and see if there is any land engagement that is visible through the marker ink. Take pics and post if needed.

[Rbartho]

Drillboss, thank you for that excellent chamber diagram. It's much easier to picture the compound throat from your diagram than from the saami drawings. My original question was if I might be getting a false positive from the oal gauge when the bullet hit the 1.5 degree taper, but I can see from your post, that is not my issue. The 1.5 taper is around the 1.75" position, and my bullets are touching around 2.245.

LRRPF52, I also appreciate your insight and suggestions. I've attempted to build and chamber the dummy rounds as you suggested, but I can't definitively tell if I'm hitting the lands or not. Hopefully you can interpret the pictures better than I can. It does appear that there are marks on the ogives, but the marks could be from the chambering and extracting process. Pics below. Left is 2.245, Center is 2.255 and right is 2.270.

Thanks,
Ross

[Drillboss]

Drillboss, thank you for that excellent chamber diagram. It's much easier to picture the compound throat from your diagram than from the saami drawings. My original question was if I might be getting a false positive from the oal gauge when the bullet hit the 1.5 degree taper, but I can see from your post, that is not my issue. The 1.5 taper is around the 1.75" position, and my bullets are touching around 2.245.

LRRPF52, I also appreciate your insight and suggestions. I've attempted to build and chamber the dummy rounds as you suggested, but I can't definitively tell if I'm hitting the lands or not. Hopefully you can interpret the pictures better than I can. It does appear that there are marks on the ogives, but the marks could be from the chambering and extracting process. Pics below. Left is 2.245, Center is 2.255 and right is 2.270.

Thanks,
Ross

Here's a thread that Joe Smith and I worked on:

http://www.65grendel.com/forum/showthread.php?11789-Contact-%97-Seating-Stem-vs-Comparator-vs-Throat

In a SAAMI chamber, the bullet contacts the lands just a little bit forward of the 123 gr SST cannelure groove.

[Rbartho]

Here's a thread that Joe Smith and I worked on:

http://www.65grendel.com/forum/showthread.php?11789-Contact-%97-Seating-Stem-vs-Comparator-vs-Throat

In a SAAMI chamber, the bullet contacts the lands just a little bit forward of the 123 gr SST cannelure groove.

After checking out Drillboss' post, I reran the test with a better idea of where to focus my poor camera shots.

The marker test seems to show the same results as the Hornday OAL gauge. The 2.245 bullet shows light marks right above the cannelure as Drillboss predicted. The 2.255 bullet shows longer/deeper marks, and the 2.270 bullet shows the entire area above the cannelure rubbed clean (as expected with a bullet seated that long)

I guess the question is whether to talk to AA about it. It would really be nice to shoot factory ammo loaded in that 2.24 to 2.245 range and still have 10+ thousandths of bullet jump.

Thanks again for the great information!

[JASmith]

That 2.245 picture is at most the "light kiss" Bill Alexander indicated works fine.

In my view, the "kiss" leaves no wiggle room and affords the most precise alignment possible. The half-degree taper means there is very little resistance to the bullet.

This suggests that yiur pressures will be fine. Adding a longer jump will likely impact your accuracy.

[LRRPF52]

Biggest thing for me is, can I extract a chambered cartridge.

Does the cartridge fall free from the chamber with gravity?

If it does not do either of theses, then I address the throat by either polishing or reaming.

[Rbartho]

Biggest thing for me is, can I extract a chambered cartridge.

Does the cartridge fall free from the chamber with gravity?

If it does not do either of theses, then I address the throat by either polishing or reaming.

The dummy round at 2.245 extracts fine. It will not fall free from the chamber with just gravity, however, Even with rapping the butt of the rifle firmly on the ground, it won't fall free. The dummy round was only seated with finger pressure in this case, and I assume that the bolt cycling would seat it even more firmly into the chamber.

Definitely seems like this chamber is on the short side of allowable tolerances.

As you probably expect, this is not a purely academic exercise. I have not been able to get the 123 SSTs to shoot in this upper. I shot 5 boxes of the Hornady factory rounds, and the groups averaged around 1.5 inches, with the worst groups closer to 2 inches.

My handloads have been a little better but only a little. I shot a bunch of 3 shot groups between 27.1 and 28.5.grains of 8208XBR and thought I'd found a sweet spot around 27.7 grains. However when I zeroed in on that load and started shooting 5 shot groups, the size opened up to between and inch and an inch and a half. Not bad but I was hoping for better. OALs for these loads were 2.24 to 2.25 inches.

After reading threads here about CFE-223, I did some testing with that powder as well. I tried 29 grains up to 31.1 and seemed to get best results at 30.8 grains and 2.24 OAL. As with the 8208 though, once I started shooting 5 shot groups, the group size opened up to 1-1.5 inches again.

As far as my abilities since that's a fair question to ask, I have a 308 Tikka that shoots .4 to .75 inch groups consistently. I will admit that I feel more "locked in" with a bolt gun than with a gas gun though. I have 30 more years of experience with bolt guns after all.

I'm not blaming the chamber of course, but it does limit the amount of tuning I can do with OAL. Mr. Stokes seems to recommend starting with a .020 jump, and I don't think it's probably wise for me to load as short as 2.225. I will say that the best accuracy I've seen has been in the 2.24 to 2.245 range as JASmith suggested it would be.

I'm still working with this upper. I really wanted the 123 SSTs to work since they look so good on paper. They have a high BC, should have good terminal performance at Grendel velocities, and are cheap enough to shoot as a paper punching round as well as a hunting round. Since I haven't seen great results, I picked up some 120gr Match Kings and 120gr Ballistic Tips to see if I can get better results.

I am also going to try a different scope and mounts and have a friend that shoots alot try this gun as well. I'm trying to look at all the possible reasons for mediocre accuracy and not just focusing on one area.

I appreciate the feedback, and I'll keep you posted with what I find.

Thanks,
Ross

[LRRPF52]

What I have seen is that the compound throat doesn't really care what COL I use, since there is a light engagement very early on with the shallow .5 degree leade.

I think something else is going on, and it is either adjusting to shooting the semi auto, or something mechanical.

Can you tell us more about the build, what trigger you're using, mount, optics, how the rings were tightened, and how you set up your position?

I personally use a nice rear bag that does't get pushed around by the recoil, or I will drive the rear bag with a lot more hand pressure than I would with a bolt gun.

When I get on a bolt gun, it feels like cheating, because I am so used to shooting accurately with gas guns, and the extra work that goes into them.

I settle the gun into position so that the reticle does not move outside of an inch on my POA on the target.

The reciprocating mass of the BCG in an AR15 on the return stroke really wants to disrupt your position, so paying extra attention to building the position is something that I find critical, especially with these lightweight guns, which like to get squirrely on you, unlike a heavy bolt gun that just free-recoils consistently in the bags.

[Rbartho]

What I have seen is that the compound throat doesn't really care what COL I use, since there is a light engagement very early on with the shallow .5 degree leade.

I think something else is going on, and it is either adjusting to shooting the semi auto, or something mechanical.

Can you tell us more about the build, what trigger you're using, mount, optics, how the rings were tightened, and how you set up your position?

I personally use a nice rear bag that does't get pushed around by the recoil, or I will drive the rear bag with a lot more hand pressure than I would with a bolt gun.

When I get on a bolt gun, it feels like cheating, because I am so used to shooting accurately with gas guns, and the extra work that goes into them.

I settle the gun into position so that the reticle does not move outside of an inch on my POA on the target.

The reciprocating mass of the BCG in an AR15 on the return stroke really wants to disrupt your position, so paying extra attention to building the position is something that I find critical, especially with these lightweight guns, which like to get squirrely on you, unlike a heavy bolt gun that just free-recoils consistently in the bags.

On your previous post, you said that if the round won't fall free with gravity then you would recommend polishing or reaming the chamber, correct? Since the round doesn't fall free with gravity alone and actually won't fall out even when thumping the buttstock against the floor, then I assume, your recommendation is to consider having the chamber reworked?

My build is a BCM lower with an ACS-L buttstock and a Geissele SSA-E trigger.The scope is a Nikon Buckmaster 4.5-14x40 AO model in a Burris Pepr mount. I set it up to manufacturers specs and torqued everything with a Wheeler Fat Wrench. I don't remember exactly but I think the specs stated like 20-25 inch pounds. Optics aren't great obviously, and I do plan on trying a different scope, but it will also be a $200-$300 scope, probably another Nikon in 3-9x, which I realize low magnification for shooting groups.

I shoot at an indoor 100 yard range so wind is not a factor. I use a protektor front bag with a cut and rounded piece of 2x6 to provide the extra height needed. The rear bag is the Protektor rabbit ears bag that holds the sloped piece of the ACS-L stock fairly well and allows me to adjust elevation easily by sliding it back and forth.

I read an interesting article in one of the gun mags about shooting gas guns that talked about pre impact vibration among other things. I have tried both holding the handguard through the shot with the bag placed further forward as well as a more "bolt gun style" setup with the bag further back and the left hand off the handguard. No noticeable difference in accuracy that I could see. I don't see the reticle moving more than 1/4-1/2 inch max off the POA before the shot. A half inch of movement would seem pretty big at 14x, and my gut feel is that the 1/4 inch number is more accurate, but then that could be ego talking I suppose.

You make an interesting point about building the position. How critical is the placement of the front bag with a free floated handguard? With all the pressure being focused at the barrel nut, it shouldn't be ultra critical to have the front bag placed EXACTLY the same for every shot, should it? There may be an inch or two of variation when I shoot this rifle.

Thanks,
Ross

[LRRPF52]

My process is to first check that the cartridge drops free with gravity.

Next I do the tap test.

Then the extraction test.

If it extracts without binding, I make a note so that I can keep an eye out for that set-up at the range when test-firing. If the ammunition is easy to extract after being chambered by the action under fire, I don't worry about it anymore. Are you having problems extracting live rounds after chambering at the range?

If that is the case with factory ammo, then it gets polished or reamed. I have only had to do that with a Satern barrel that had a short throat.

As to the position, if there is considerable change to the position that your handguard is indexing that far back with each shot, you need to lean into the rifle with your shoulder and keep it as static as possible.

One thing I find helpful to do that is using a bipod in the folded position on the front bag, so that it isn't going anywhere.

I have had problems with people's Burris PEPR and Nikon mounts with the cross bolt stripping out when properly torqued. The teeth on the opposite side that are supposed to stay secure in the aluminum base strip out the cheap aluminum, which is why I personally like the NightForce Unimount so much.

How much torque did yo use with the cross bolts on the PEPR?

[Rbartho]

LRRPF52, thanks for your help and for your patience with my questions. See answers below.

[QUOTE=LRRPF52;132685]
Are you having problems extracting live rounds after chambering at the range?

It's not something I do alot, but I think live rounds extract OK as well


As to the position, if there is considerable change to the position that your handguard is indexing that far back with each shot, you need to lean into the rifle with your shoulder and keep it as static as possible.


I'll give that a try, thanks.

How much torque did yo use with the cross bolts on the PEPR?[

It's a quick release mount so I didn't torque the bolts. I opened the levers turned a quarter turn and closed the levers and repeated that process until they would just barely cam over into the closed position with finger pressure.

/QUOTE]

I have a vortex diamondback hp 3-12x42 scope and a larue lt-104 qr mount on order. I'll mount them on the grendel and hit the range next weekend. Would be awesome if the groups tightened up..

Ross

[LRRPF52]

With the QD scope mounts, I have found that you really need a lot more tension on the levers than you think, to the point that they are no longer QD/QA.

When the levers hit 45*, there needs to be a lot of resistance, to the point that you really need to bear down on them with what some might call excessive force.

I've seen a lot of them come loose over the years during courses and competitions.

On the rings, I normally only use 15-17 inch pounds of torque, balanced out across all the screws.