The weather forecast for my sheep hunt makes my thinking return to a question I've never fully answered for myself.
When rain or condensation makes the inside of your barrel wet, how much does point of impact potentially change on a five or six hundred yard shot? I may try testing that at my range before I leave but do you have any experience?
There is no easy answer to your question and there is also no easy experiment to determine the answer. In my experience and testing I have found that the temperature of a "cold, wet barrel" has more effect than the moisture. There is a point where the moisture will become significant. Moisture in the bore near the muzzle seems to have little effect. Moisture close to the chamber can be dangerous. Different barrels seem to tolerate moisture, with little point of impact change, better than others.
If it is raining heavily then I use a piece of electricians tape over the muzzle. I have also used one of those small "rubbers". The air blast in front of the bullet will blow this off. If you use this method you have to take the tape or rubber off about once a day for several hours or run a patch down the bore, as condensate will build up rapidly inside.
The mountain guides and many Sask deer outfitters that we have hunted with like to put a wrap of electricians tape over their muzzles. Cut a piece about 3 inches long and stretch it over the muzzle, then cut little slits in it at right angles so the tape will conform to the barrel curve. Then put one more wrap around the previous piece of tape at right angles, all around the barrel about an inch below the muzzle. I have shot the rifle on the range and cannot determine any differences in POI or accuracy.
We also but about 4-6 inches around the barrel at the midway mark so that we have some replacement tape in case we fire a shot. Tape doesn't stick very well in real cold weather but putting the second piece on will hold the original in place.
The little rubbers are good too no doubt, but electrician's or duct tape is usually in camp.
Interesting you should ask the question as I was going to post something to that effect, only with oil left in the barrel.
When shooting 1000 yard competitions and leaving the clicks on the scope from the previous match, I have found that, the first shot out of a clean barrel (only a small amount of oil still left) will be low on the target as much as 2 and sometimes 3 feet.
This is especially true using a large weight bullet such as the 300 Gr 338 fired from a 340 IMP case.
Without any change what so ever to the scope, the next shot will be UP in or very close to the bull. The fifle needs a "Fouling" round whenthe least amount of oil is left in the barrel. Just running a dry patch in does not always clean it out.
The same holds true in our Longrange hunts. We always make sure the rifle is fired a few times before we go after an animal.
When any oil is left the barrel, the resistantance normally given to the bullet from the bare clean lands and barrel steel is not there. Hence, a fouling round or two should always be made when shooting LR.
The bullet exits too rapidly without the correct amount of pressure applied and all the powder does not have a chance to burn while the bullet is still within the barrel.
Almost like when swithching to a Moly bullet, you must go to a faster burning powder.
I have seen this in most all calibers when shooting LR.
To clear that up a bit, most centerfire rifle bullets will have reduced velocity coming out of a wet oiled barrel for the first shot or two. Thats why you will see the bullet impact LOWER down range.
Just because a bullet SLIDES out of a barrel easier because of less friction doesn't mean the velocity is increased. It only means it started it's trip down the tube at the first sign of pressure from the START of the powder beginning it's burn. The bullet was not held back enough to obsorb all the pressure from the powder burn that creates velocity.
The cronagraph is very helpful in this senario.
AS the fouling takes place and the friction is applied to the bullet from non oiled lands and grooves the increased load pressure will cause the velocity to increase and your bullet impact will be higher and shoot flatter down range. Hence, an increase in OVERALL bullet velocity on the path to the target.
On another note;
I believe I read where you were working on a GPS rangefinder system.
Am I correct in my asumption that, you must take a reading from the shooting position and another reading at the target area you intend on shoot to.
If this is true, would you not have to take readings in several places on THAT mountain to know the yardage in case an elk or deer showed up somewhere in the area you plotted?
I don't believe this would work unless you hunted the SAME mountain all the time and had plotted it in advance. If you went to a new place, you would have to plot the whole new area again.
Maybe I'm confused or did I miss something? .
The GPS I have used gave a reading as to where you are and can get you back to where you started by reversing the coordinents.
I never thought of it as a rangefinder because we need to be able to range the animal in a matter of seconds or a minute or two at the most. Normally the animal is moving slowly and we need to get the range and set up on him as soon as possible.
The military lasers we use are excellent and very accurate for this.
Yes, the GPS works as a ranging instrument but as you suspected, you must get a position fix at the intended impact site.
The GPS will get a good fix to within 5 to 7 yards on both ends of the flight path.
The program Peter Cromhelm has on his site, the "Thing", lets the user enter multiple positions where an animal/whatever may present itself. It also has a place for the position of the shooter. By using these positions the program gives distance and direction to the various target positions. You can also enter the wind direction and value from the shooter position to one 'main' target site and the program calculates all other wind values based on the single reference wind. (Assume a single source/direction wind.)
It's a fairly simple thing, I hunt in areas where I know the animals will enter the fields and clearings at a known location. I pre-fix these locations with the GPS. I also can select any location I desire as my firing position. All I need to do to get a target distance once I've setup my area is select my firing position and the correct critter position, the program has the distance, come-up data and wind setting (providing I've entered wind data).
It's probably more value for a varmint shooter but works well for me for deer.
I believe Peter has the program on a palm computer and takes it into the field with him. I've used it on a palm type HP Jornada handheld computer and it functions great but the LCD isn't backlighted so it's nearly impossible to read in bright sunlight. A backlighted LCD is pretty much a requirement for the handheld, the Casio E-115 Cassiopea is a backlighted model.