Author Topic: POI rising with heat (Read 676 times)

SeaBass · « **on:** July 25, 2005, 06:41:06 PM »

I have a Weatherby Vanguard in .270 Win that I have free floated the barrel (crappy Butlercreek stock) on. The first three shots from a cold barrel group between 1" to 2" at 200yds depending on how well I'm shooting. It will even shoot <1" at 200yds if I'm on my game. My handloads are probably just a hair hotter than factory loads.

The problem is with it being so warm out, it takes a very long time for the barrel to cool. it seems that each group of 3 I shoot when the barrel is a bit warm impact a little high maybe 1/2" or so. Groups are still sub moa but the POI seems to continue to climb as the barrel warms.

I shoot 3 rds with no pause other than loading and aiming. I wait 8-10 minutes and the barrel is still warm but not hot. I shoot another group of 3 and it's 1/2 high. It's not me (I thought it was) because it will happen consistantly.

I check the stock for high points with a dollar bill between groups and it seems to pass freely.

Should I try inletting my stock some more. Could this be the result of heat mirage off the barrel?

Today was 80F with a very light cross wind from right to left.

The scope is a Mueller sport dot 4-16x50 AO. The adjustments are crisp and predictable. I believe the scope is holding zero very well.

Is this normal or do I have a problem?

Dave in WV · « **Reply #1 on:** July 26, 2005, 04:13:15 AM »

I'm not a gunsmith. IMHO it's the barrel. Some barrels will do just what you describe as they heat up. It's my understanding that's a primary reason to "cryo" a barrel especially when the shift is greater. As irritating as your situation seems, to me it's not a problem. Looking at it in a practical light, how many shots do you take when hunting? If your rifle holds a good zero it wouldn't be a problem to me.

Shorty · « **Reply #2 on:** July 26, 2005, 12:30:52 PM »

SeaBass,
1/2" change of POI at 200 yds?
80 deg. F today?
I wish I had YOUR problems!

:grin:

760 Nut · « **Reply #3 on:** July 26, 2005, 03:37:56 PM »

You can expect POI to change as the barrel heats up due to thermal expansion. As the barrel heats up the o.d. and i.d. of the barrel will expand (as well as the length). This can cause pressure on the barrel as well as the bore being a bit more "sloppy". It may only be a few thousandths of an inch, but it's enough to effect POI. The coefficient of thermal expansion is different in all grades of steel, and thus rate of thermal expansion may not be the same from various manufacturers barrels.
If hunting, the first shot is the most important afterall, right?

SeaBass · « **Reply #4 on:** July 26, 2005, 06:43:32 PM »

Thanks for the replys guys. I was thinking along those same lines myself.

However, after my trip to the range today I do not think the barrel is the problem. I fired three shots from a cold barrel and the POI was exactly were the last group I fired yesterday was. The groups from yesterday climbed about 1/2" with each group fired. with a cold barrel they should have been right in the bullseye. I adjusted my scope to hit the bull and fired 3 groups of three. I let it cool between the first and second groups and fired the third group right after the second. The barrel hot. I hated to do it but I needed to know what was going on. Well all three groups hit center mass and the last/hottest barrel group was the best at 1.5" @ 200yds! Today was 86F with a bit more cross wind.

I think perhaps the aft screw on my stock is too tight and it was drawing the action farther into the stock with each shot??? Or maybe I have a high spot on the stock I'm just not finding. How easy should the dollar bill pass under the barrel? Do I need to push down on the barrel when I check it? How tight should the stock/action screws be? My front screw isn't terribly tight but I do tighten the aft screw pretty good. If I knew what it should be I would use a torque wrench on them.

Thanks for your help!

P.S. a couple notes of intrest.

The best groups I ever fired with the rifle were with washers sitting between the stock and the action when experimenting to see if I should free float or not. .96" to 1.5" 3 shot at 200.

I have target turrents on my scope and I checked them after each shot today to make sure they weren't moving. Its possible they moved yesterday since I didn't have the settings written down. I do now and will be looking to see if they change but they don't seem to be.

gunnut69 · « **Reply #5 on:** July 26, 2005, 07:44:02 PM »

It's likely the action settling into the tupperware stock.. I'd deep six the junky plastic and replace it with a good laminated model.. they're as stabil as plastic and warm to the touch.. It may even be the plastic stock moving as the rifle's fired. I dislike sybthetic stocks and cheap synthetics are worst of all..

SeaBass · « **Reply #6 on:** July 26, 2005, 08:02:27 PM »

Hi, Gunnut69. Thanks. There is definitely a new stock in my future. I would very much like a laminate stock but was also considering a bell carlson stock. Any recommendations for a guy on a tight budget?

Also after reviewing my targets this past weekend, I noticed that its not a rising POI its a shifting POI problem! I don't understand? How can my POI can shift (drop in this case) from one 3 shot group to the next when when the first group was 1.38" and the next was 1.25" at 200yds?

It seems like any problem that would shift my POI would open up my groups? I break my cheek weld every time to load a new round as I only load one at a time. The groups were less than ten minutes apart. What could change between groups but not affect groups in progress? I would be quick to blame myself but I was shooting very well? Could the rifle settle into the stock as it cools/contracts? But not as it warms up?

SeaBass · « **Reply #7 on:** July 27, 2005, 01:17:56 PM »

Happy Birthday to me! Just picked a custom made laminate stock. Wish I knew how to post a pic of it. Bye Bye Plastic stock!!

gunnut69 · « **Reply #8 on:** July 28, 2005, 04:27:26 AM »

There are too many variables in a group to say it's a unique entity. Numbering shots in a group as they are fired may help but in reality it likely won't.. If we add the shooter error, the fact that bedding can shift a little at a time or a lot and even things like chance(luck) there's simply no way a single group is in any way difintive.. Several groups alike can be 'indicative' but still not definative.. Cheap plastic stocks are terrible, the good ones are quite good.. Laminates have some strong advantages and are not that expensive... I would suggest perhaps a little less emphasis on a single group and a minimum of 5 three shot groups indicating a trend..

Big Paulie · « **Reply #9 on:** August 02, 2005, 09:39:03 AM »

As folks have mentioned, you are probably wasting your time trying to wring target gun accuracy out of a cheap plastic stock. I am frankly amazed that you are getting such good groups.

I like the Bell and Carlson top of the line stocks. I have paid big money for HS Precision drop-in stocks, with the pre-formed aluminum pillar bedding, and have found them to be no more accurate than the good wooden stocks that I was replacing. The Bell and Carlson were not as expensive, and gave me about the same results. Also, they seem to be noticeably lighter than the HS Precision.

Be prepared to pay good money for a synthetic stock. You have already had the experience of dealing with a cheap one.

Best Regards,

Big Paulie