K98 Down Grade

right i'm going to pretend i understand all that

so butane maybe the right root as long as i regulary lube the seals

Both are flammable hydrocarbon gasses but not the same. A bit like peas and broad beans - both legumes but very different.
Wikipedia:
Propane is a three-carbon alkane with the molecular formula C3H8, normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central heating.
A mixture of propane and butane, used mainly as vehicle fuel, is commonly known as liquefied petroleum gas (LPG or LP gas). It may also contain small amounts of propylene and/or butylene. An odorant, such as ethanethiol or thiophene, is added so that people can easily smell the gas in case of a leak.
Butane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to either of two structural isomers, n-butane or isobutane (or "methylpropane"), or to a mixture of these isomers. In the IUPAC nomenclature, however, "butane" refers only to the n-butane isomer (which is the isomer with the unbranched structure). Butanes are highly flammable, colorless, easily liquefied gases. The name butane comes from the roots but- (from butyric acid) and -ane.

Ahh, now that explains everything

Yeah stupid Wiki - all they need to say was Propane has a higher fps than Butane.

Mapp gas can be used too especially in very cold weather, that's like the propane from the plumbers merchant but in the yellow tins. This is also not for Marsha as gives even more fps but I run a GBBR and mapp gas worked fine, even for short bursts of full auto at sub zero temperatures. I wouldnt run it through normal gas guns in the summer, it would probably destroy them. In a K98 you'd be likely to be hitting stupid high fps on a hot day.

in essence different gasses will give different power and lower air temperature will give less power and higher air temperature higher power,so to get the same power on hot or cold days will need different gasses but you will never or almost never be able to tune a gas gun as exactly to say 350 fps by changing gas as you can an electric gun by changing the spring witch is why its usually used in pistols that are less likely to be overpower except on hot days or on sniper rifles where you are looking for over 400 fps anyway.all this eventually leads to needing your own chronograph to see what works while messing about at home when you have time rather than at a shop or local club,they dont have to be expensive ,i got a goodish one from rsov for about £35 plus post.

Marsha, I have the same need as you wanting to reduce the Kar 98 to 350fps for certain games (Patton Stalled) to avoid the need for minimum engagment distances. Tanaka did produce a reducer for this purpose but I have not been able to get hold of one.

On Dadio's advice I have just tried butane, the same type as we use in our mini tent heaters, and it chronoed at between 335 and 355 so that definately seems to be the way to go (Even 134a was chroenoing at 370)

right i had it chrono again but at JD airsoft this time and it done 520fps on 0.20g max never went over that even with 8 shots
no adjustments were done i used the same gas predator ultra so it may not have been chrono right before
but what ill do is still get a lower gas and get a chrono so i can regualy check the fps myself