My first project with pistol reloading will be loading my newly acquired 686 4"
I have a 4" 686; as well as a 3"; and an 8-3/8".
I load/shoot a lot of cast (hard cast, to be exact) 158's for 357 Mag. My pet load is 8.2gns HS-6 with a CCI 550 magnum primer. It runs 1145 f/s through the 4" bbl. Lead likes HS-6 because it runs cool. If you can find the window where it's hot enough to not run too sooty, yet tame enough to not cause major leading issues, HS-6 is a good way to go. Speer #14's top load is 9.7gns, to give some indication where 8.2gns is. It's mag level, but not roaring. Assuming your measuring technique is accurate, you could go straight to the 8.2gns - no need to "work up" to it. Need the magnum primer with 357 Mag / HS-6 so it burns consistently.
125g JHP. 38 and 357 varieties.
For 125gn JHP / 357 Mag, I have a few loadings. Unlike the above-mentioned lead SWC loading, you'll need to do your load work ups with these:
125/357: 9.7gn Power Pistol. This one runs 1277 f/s through my 3"; so figure about 1340 thorough a 4". Speer #14 peaks at 10.5gns, but this is where they ran most consistently (low Standard Deviation - 11.42) in
my gun.
125/357: 11.0gn HS-6 (magnum primer). Runs 1342 f/s through my 3"; so figure about 1410 through a 4". This is a good, potent round that shoots straight, but shows a lot of Standard Deviation (20.42), so I don't load it much. HS-6 prefers heavier bullets (my 158 JHP round with HS-6 is excellent - 9.0gns). But it's still a great shooter and I wouldn't discourage you from doing a work up with this bullet/propellant combination. Speer #14 peaks at 11.3gns, and I've been there; but SD's continue to increase after 11.0gn.
125/357: 9.2gn Unique. Runs 1312 f/s through my 3"; so figure about 1380 through a 4". SD 12.20. This is a good, economical range shooter, with enough pop for recoil practice. Speer #14 peaks at 9.6gns and I found rapidly increasing SD's after 9.2; so 9.2 it is.
I dont own a chronograph to check my loads so how do u guys test which load/powder is best?
With a chronograph
Get one.
Points to ponder: You have 2400 and 4227. Both great propellants. But I would discourage their use with 125gn bullets (others will disagree, no doubt). I think they're too slow for the bullet weight - especially through a 4" bbl gun (maybe 6" or 8", maybe). Yes, you'll squeeze more velocity than my above-mentioned examples. But what you'll get in return is inconsistent results (high SD's) and a
lot of muzzle flash, report (noise), and recoil. If you want to get a lot of attention at the range, go for it. The youngsters will be impressed; the old-timers will be laughing at you inside their heads.
Consider - strongly consider - getting some 158 jacketed bullets. I have found that 158's run far more consistently in short barreled guns (<6" in the context of this discussion). And they're also inherently more accurate at normal handgun distances. I also prefer the recoil - stronger overall, but less "snappy."
Furthermore, consider that the velocities of the bullets of the above mentioned examples are right were 357 hollow points are designed to operate from a terminal ballistics standpoint. i.e. they expand most thoroughly and reliably around 1200 - 1400 f/s. More than that, and they start to break apart and can actually be counter productive. I don't want to morph this into a discussion of terminal ballistics; but I want to impress the point that "good enough is good enough." I like using things in a manner in which they were intended. I've found over my many decades of life that engineers often get it right. If you're running your JHP's around 1300 f/s, you've got them in their wheelhouse.
125gn JHP's with 38 Special:
38 Special SAAMI compliant pressures can propel 125's to about 1000 f/s - that's
just getting fast enough for expansion. Now, if you're going to shoot them through a 686, you can load above spec pressures and be fine. But I personally, won't do it. If I want more juice, I'll move to a 357 Mag case/data. So all my 38 Special loadings are published data compliant - and therefore, by extension, SAAMI compliant.
Because of the lower velocities, I rarely load an actual jacketed bullet with 38 Special. But I often load plated, and their data basically crosses over to jacketed (i.e. if you substitute my plated data with a jacketed bullet, you'll likely get similar results).
My favorite 125/38 Spl loading is 5.7gn Unique. 999 f/s through a Smith model 67 (a 38 gun) w/ 4" bbl. Although this round has a bit of SD (21.11), it shoots nice and straight and has a "good recoil" (purely subjective) and just feels right for
my gun. Speer #14 peaks at 6.0gns.
I have a ton of 38/158 LSWC recipes. 3.5gn Bullseye is a real beauty. 800 f/s through my 4" 38. Shoots extremely straight with a 10.86 SD. This is likely the best application for your lead 158 SWC's.
For 38 Special, I do not use HS-6 at all. I have never gotten good results. HS-6 just prefers more pressure. I suppose you could get it to clean up with 158's, but I have lots of other more suitable powders, so I have no reason to (re) try.
For 38 Special, I only use Power Pistol with 158's; and rarely at that.
I have no recipe for Bullseye/125 JHP. You can make good, consistent range shooters with Bullseye/125 JHP, but it's not very economical. Most would just move to the lead 158's.
Hope I gave you enough to chew on
. For 38 Special, the published COL is 1.455". It's not that critical; as long as you don't load them too long to fit in the cylinder; or so short that you run into pressure problems. Anywhere in between is fine - as long as you keep it consistent from load batch to load batch. All that said though, go with 1.455" 
