My brother and I are finishing our sight in for our upcoming muzzleloader hunt with our NEF Sidekicks and a NEF Huntsman

Had the preliminary sight in work done with 2 Sidekick and a newer Huntsman, all 50 caliber, with UnMentionable breech plugs. Had these 3 sighted in with the 3 holdpoint Cabelas scopes. Only difference today was colder weather here at 32 degrees vs 68 degrees last time we shot. (first time doing extensive shooting in colder weather) The shots hit as much a 10" lower today. Only thing that brought the group up somewhat was switching to CCI 209 Magnum primers that I've had forever vs. Federal 209A that I've always used

Have always used 84 weighed grains of Blackhorn 209 with Cecils Dead Center 200 grain bullets just like last year and the lot # of powder and primers are the same also. Have always used weighed charges but notice the volume can change from 111 grains to 115 grains in volume by weighing them vs pouring a 120 grain charge by volume.

Anyone notice that temperature changes can really affect Blackhorn 209 powder? Any other ideas?

nope, 80-90 to 1* and they hit exactly where it was originally sighted in for. Sounds to me like your breech plug is not up to specs.

Have shot it from around freezing to 90 degrees, no changes. As a matter of fact, shot it today, first time in almost 2 years, still dead on. It was about 55 today, about 85 last time I shot it.

I have heard 209 likes a lot of seating pressure, alot. Before I read that, I could not get any decent groups. I really bear down on it with a range rod now and it makes a huge difference in my gun.

This has been my experience as well. A lot of seating pressure...more than you think is necessary...and my groups shrunk by half.

I will try laying on the ramrod a quite abit more when seating a bullet and see if that helps. I was also thinking that the gas seal of the sabot might be lacking when the weather cooled off.and the sabot contracted a bit.. Will also change the Oring in the primer pocket every 5 shots or so. I am going to look at everything I can. Even purchased two more brands of saboted projectiles to try. Thanks for the tips, I'll let you know what happens.

From about 75 degrees down to 15-20 degrees with no trouble. Seating pressure will change where your shots hit. I started using quite a bit more force when seating and things really settled in.

UPDATE*****To make a long story shorter I consulted with a friend who is a muzzleloader guru and with his reply I eventually figured that probably two of my identical lot numbered 4 pak of Blackhorn 209 came thru with seals that leaked air and one for sure that I have yet unopened has a leaky seal on the closure cap. My friend told me to do the squeeze test and sure enough one container of powder would take air out and in when squeezed slightly.

Being that I weighed my charges I had to increase the weight of my charge to get my impact back to where it was. I had no issues with the windage.

What I deducted from consulting with my friend, was that one of my remaining two unopened containers is not sealed. To figure this out I weighed an empty can of 209 and it weighed 2 ounces. One of the unopened containers weighed weighed 12 ounces (No air escape)and the other (a leaker) 12.8 ounces respectively. (They are filled exactly with 10 ounces of BH209 at the factory). So I deducted that this powder had taken on moisture to weigh in at 12.8 ounces.

From now on I will use a decent measure and do only volume charges doing every load the same way with the same number of taps to the side of the measurer each time to get to my load, seeing that weighing them may not always work.

That in my opinion was the biggest culprit I was facing. Took awhile but thanks to a friends advice, I finally figured it out.

BTW, I killed a nice 4x5 muley at 206 yards with my 24" 7/8" MU plugged Sidekick last Saturday morning using Blackhorn 209 and a PR bullet.