Ok, here's my contribution to the XBR discussion.  

I just tested XBR in my AR.  It has a 20" stainless bull barrel with a 1:8 twist.  I shot 5 round groups ranging from 21 grains to 23.8 grains in .3 grain increments.  This covers the entire published range.  The only signs of pressure appeared at the very high end of the range in the form of very slight ejector marks.  No other pressure signs were observed.

Results:  I got good accuracy(7/8") at the very low end of the range(21grns), but as the charges increased, so did the size of the groups; and all over the target too.   Normally I am happy with a 7/8" group, but 2,469 fps seems to slow to be considered a good load.  

For those of you wondering, my average velocities ran from 2,469 fps to 2,809 fps.  The IMR published velocities are 2,696 fps - 2,959 fps.  For comparison, I ran some Black Hills 69grn SMK factory loads over the chronograph and got an average of 2,706 fps with about 1" groups.  

I'm kind of scratching my head as to what to do next.  I'm happy with the accuracy of the lower end load, but would love to have some more velocity for longer range shooting.  Everyone says that accuracy trumps velocity, but how slow is to slow with 69grn smk's?   Plus, everything I've read seems to indicate that people are finding accuracy at or just over the published max.  So, should I carefully keep working up hoping to find another accuracy node, or should I just be happy with the slower load??  

Also, if I find an accuracy node at 2,400fps is there a way to estimate at what velocity the next one will appear?  I'm guessing the answer is no, but this sure would be helpful information.

UPDATE.......I loaded up some more test loads ranging from .6 under the published max to .9 over the published max(23.2grn - 24.7grn).  Even at .9 over, the only evidence of pressure was light ejector marks which are pretty much evident on every case I fire from this gun.  I found good accuracy at 23.5, 23.8, and 24.1. located dead center of the target.  At 24.4, I had a really tight group, but it was about an inch high and left.  At 24.7, the group scattered again.   I've settled on 23.8 grn which resulted in a 3/4" group dead center, or 1" counting a flyer that impacted just high.  

Conclusions:

Like others, I found the best load to be right at the published max, or maybe .2 over.  I saw little evidence of pressure, so I believe that I could safely work up even hotter if I so desired.   XBR meters well enough to load progressively and still maintain good results.


Brass= WCC10
COL from Ogive = 2.035
COL = 2.235 - 2.240
69grn SMK
CCI 400
23.8 grns. IMR 8208 XBR

Is that your best advice?

What is your case length and overall length of your load?

What primers are you using?

When was the last time you cleaned your barrel?

Case length =1.755"
COL = 2.033 from Ogive  which is close to a 2.235" COL
CCI 400
I started with a clean barrel and ran about 10 through it to sight it in.  I shot the groups in Round Robin fashion.

I shot them in Round Robin fashion so I would think that if the number of rounds through the barrel were a factor it would have also have affected the first group.

It may be too fast for the 69 grain, I don't know.  I think you suggested that in another thread.  I've also tried Varget and BLC2 with similar results.

I honestly started to question the barrel, but the tight group at the lower end of the range would seem to indicate that it is capable of accuracy.  That's why I asked if anyone knew where I might find the next accuracy node.  Maybe it's above the published max?

 

Since I am seeing no real signs of pressure, I am going to work up a load at .3 and .6 over max and see if that gets me into an accuracy node.  And yes, I am always very careful when working near max.

What barrel length and what velocities are you getting?  Any clue?

 

I get very narrow but very accurate nodes out of XBR.  

I've used it in .223 and .308 with great success.

For instance, with 50gr VMAXs, 25.2gr and 25.8gr get me 1.25" groups. 25.5gr gets me 0.5" groups consistently.

The velocities you can get safely with XBR are outstanding.

I've done a lot of testing in my 18" 1/8 AR and 18" 1/7 AR.  I have not found a crazy "tack driver" 1/2" moa 69SMK load as of yet...and I've gone quite a bit over book max.  The loads are good enough for my use at this point but I'm going on to try other bullets out, namely Hornady 68gr BTHP and the Berger 70gr VLD.

With 26.0gr of XBR with a 69SMK I get right at 3000 FPS avg velocity out of my 18" 1/8 BCM barrel.  That is sick!
 

Stainles 1/8 ................................... Wylde chamber?

I found an improvement from 2.255 going shorter to 2.245.


2.235 may be a bit tight.


My reasoning is that seating depth affects the pressure in the cartridge.

and people report Black Hills factory 77SMK to be ~2.245

26g of XBR under a 69??

I have read that it's hard to get over-preassure with this powder, but i am definitely not going to run right out and try that load.  I will however work up slowly past the published max  and see where that gets me.  I know I can safely do that.

Thanks for the info.  Interesting about the narrow bands of accuracy.

Yes.  Be careful.  It's hot.

I've only tried 155.5 Bergers in my .308 with XBR.  46.8-47.0 gr produce very accurate loads.  Much to either side of that, the groups open up bigtime.

I've heard that while the published loads for XBR tend to be conservative (at least for 5.56/Wylde/Noveske chambers), once over-pressure hits that it spikes fairly quickly.

Listen to this guy-your pressure increases when the volume of your combustion chamber decreases.
Also, depending upon your throat and bullet seating depth the bullet must "jump" from the case to the lands of the barrel.  When the bullet hits the lands this can cause a spike in pressure as well.  A lot of benchrest shooters using bolt actions will seat the bullet in the case so that it is just off the rifling in the throat.

For AR's seat the bullet out as far as you can and still get it to function/feed in your gun.

Is use a JP LMOS carrier, adjustable gas block, carbine buffer.

The only sign of high pressure I get are slight ejector smears.  I get similar ejector smears out of run of the mill XM193.

I'd try a different primer, and or change the seating depth.

I have considered the primer being a source of the problem.  I bought a 1,000 of the 400's because that is what was available at the time.   I use BR-2 in my .308 and have had good luck.  

Can a primer really make that much of a difference?  I'm truly wondering this.  

I've loaded a new batch from .6 under max to .9 over.  I will of course carefully work my way up.  I started .6 below because although I didn't get tight groups there, they were centered around the bullseye which is where I was sighted in at.  Most of the other groups were high and right of the bulls eye.  

We'll see I guess.  

I found up to a third of an inch at 100 yds. And velocity/burn can be "different".

I ran over book max today with Berger 70 VLDs.  Pressure signs showed up a lot earlier using the Bergers compared to the SMKs.

Tried Hornady 68 BTHP as well.  They have a lot more uniform meplats than SMKs it seems.

Still haven't found a SUPER accurate load that is running 2900+ FPS where I'd ideally like it to be.

Will try more tomorrow if the weather is decent.  

Hodgdon will be out with the new SUPERFORMANCE powder after 1st of year.  That might be a better powder for what you are doing.

Every load I've tried in this gun runs about 1 1/4" to 1 1/2" so a gain of 1/3 would put me nearer to satisfaction.  I would be happy with consistent MOA results.  That's really not asking that much I don't think.

I have found pretty much what others have reported, that with the 69gr smk the accuracy came at close to the max published load for XBR in my RRA NM 20" barrel.  At 23.5 grs. of XBR I am getting about the same velocity and accuracy as my 24.5 gr. Varget load for same bullet.  At 24.0 grs. of xbr the groups started to open back up, and I did not bother going any farther that day.

69 gr smk
23.5 grs. IMR 8208
Federal 205M primer
LC case 1.750
oal 2.260
2,700 fps./.640" moa

Hodgdon's published velocities seem a little high to me for this powder, even for a bolt gun with a 24" barrel.
I am still not real excited about it for the 223, but it has become my go to powder for 175 gr smk in my 308.

I shot these at 600 today.

77 SMK
23 grn XBR
Wolf SRM
LC mxd
2.245 COL


I was 54 clicks up from mechanical zero. My mech zero is 5 clicks off the bottom of rotation. RRA A2 sights.  eta: 1/4 moa sights

this was my first legitimate time at 600 with any powder so I don't know how that compares.

That seems like a lot of clicks.  published known come ups for getting on paper are listed as 16 moa from 100 to 600 which is 32 clicks on my 1/2 moa NM sights.  With a 77 smk over 24 grs of Varget I am up about 5 clicks for a zero at 100 and about 39 or so for 600 depending on light and weather conditions.

100 to 200 = 2 min.
200 to 300 = 3 min.
300 to 400 = 3 min.
400 to 500 = 4 min.
500 to 600 = 4 min.

Mine are 1/4 moa sights. So I guess thats 13.5 moa from mech zero.

I now added that to my prior post

That makes sense, maybe you are 13.5 moa up from your 200 yard zero.

Well I counted backwards to the bottom to be sure, came up 5 clicks, and I was on my mark for mech zero.

I shot again today at 600 and that total elevation held, 13.5 moa from bottom

eta:  I made no elevation changes from the 200 to the 300

Ok guys. I read on a different forum of some different loads being used grain wise with 8208 XBR

So I went to reloading room where a have a test drum. I use test drum to test loads that I make for pressure signs on rds I load and weapons function.

I loaded 8 rds using IMR 8208 XBR to check for signs of pressure. Here are the loads

CL 1.750
OCL 2.250
77 gr Nosler Match HPBT
WCC once fired brass some dated from 1978
CCI #400 primers
Light Crimp
18.5 in Douglas match barrel midlength gas

Now IMR list 23.2 as max load for 77 grain SMK at I think 2760 fps

LOADS WERE

23.5
23.7
23.9
24.1
24.3
24.5
24.7
24.9

There was absolutely no signs of pressure what so ever. Primers were still round and had not flowed in pockets, heck the PMC M855 flatten the primers more,  no bolt face marks, no hard extractor marks on head rims, no cratering. I found nothing and I used a magnifying glass to look at brass. My brother-in-law check the brass also. The reason I did this test was that on the other forum a couple guys loaded up to 25.0 grains using an 80 grain match bullet with no signs of pressure. So I tried some loads to see how high I could go before seeing over pressure signs. I did not see any. I did not want to press my luck though and keep going

Now I AM NOT SAYING THESE LOADS WILL WORK IN YOUR WEAPONS for my weapon there were no pressure signs and with this powder not being heat sensitive I'm loading 5 rds of each to test at the range Thurs.

I've read multiple accounts of going way over max with no signs of pressure.  I wonder if it just spikes throug the roof somewhere along the way?

Good question.

I loaded 22 to 23.5 and each primer looked exactly the same; 'slightly flattened'

The same way a factory round turns out.

My best grouping was around 22 and 23. So far I'm staying at 23 for velocity

I really don't see how it could unless the powder was very heat sensitive or the powder was messed up at the factory. This powder is not suppose to be heat sensitive. I let some TAC and some factory ammo get hot in the sun a time or two and could tell a difference. That being said I'm only going to load up to around 24.5 to see what kinda of groups I get. I kept waiting to see some flatten primes or something but never did and I did not want to load any higher than 24.9. I loaded 120 rds this past week using 77 SMK and 75 Hornady using TAC and 8208. At 100 yds with 23.8 gr of TAC I could cover a 5 rd group with a dime with the 77gr and 75 gr loads was about .75

TAC 23.6 - 24.5 and 23.8 was te best load for me
77 SMK
CL 1750
COL 2250
CCI 400 primers
LC brass

75 Hornady
same as above except COL was 2245 all had a light crimp

Loads with the 8208 were from min to 23.5
COL 2250 and 2260
CL 1750 and 1760
I tried several combination 5 rds each for a total of 60 rds in the 8208 and 60 rds for TAC
I could not get as good of a group as I did with the TAC. So I will try these higher loads Thurs.

I would love to try some TAC, but can't find it anywhere locally.  I can't justify the hazmat fee for just a lb or two to test with.  I should have bought a bunch of it when Midway had it on sale, but I had just made a large purchase and couldn't stomach spending any more money.

ETA...I am still hoping that this new XBR will be just as good as TAC .

Not a huge TAC fan here.  I mean, it isn't bad...but it isn't that good either.  It'll delivery high velocities though.