Excellent points, particularly about converting a Saiga. I tend to forget about that because I prefer the full 74 package like the 105 or SGL31 but converted sporters are very functional and IMO a better buy than the lower end kit guns. $650 will do the conversion nicely, but it could be done for about $100 more than the budget 5.45's which to me is totally worth it.
eta: I don't know about them being unobtainium...rare yes but as I mentioned I've seen three of them go fairly recently. GB is not the place to look of course and you will definitely see the $1000 priced ones in the EE also, but with patience the good prices should come along at some point.
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OTOH for me spending $150 more than a used 105 to get to an SGL31 isn't worth it, in my honest opinion the SLR105 is a better built rifle all things considered.
I have the original SGL31-44, an SGL31-61 and my old and trusty SLR105 from 2005. The 105 is a nicer rifle, it's butter smooth, everything fits and it field strips easily. As I've said before the SGL31's are much tighter and stiffer by comparison. My 31-44 is actually somewhat of a PITA to field strip and despite how many rounds I've put down range it isn't loosening up much. The 31-61 is breaking in and doing much better, but neither are as well fitted as the 105. That being said you can't argue with the Russian pedigree which is important to me...who can argue with owning a semi auto factory built AK74M? Worth every penny, but I still believe the 105 is better built.
Granted the 31-44 was the first 5.45 SGL to come in and my 31-61 is older as well, so maybe the newer batches particularly the 31-94 are smoother and better fitted.
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Z