I use lm317 all the time for single mode and multi mode laser diodes, works very good, that said the BDR-209 can be sensitive around 500ma, why not use a Sharp GH0406AA2G you can get near 2W and it looks good with a G8 lens in the sky.
LM317 works the same as LM338 only the 338 will do up to 5A
NOTE: You need enough voltage for your laser diode + 2 more volts for the driver so diodes that need 6V or more need 3 battery in series. Some LM317 are " low dropout " and can need only 0.5V extra.
You divide the constant 1.2 - 1.25 by your resistors ohms to find the current.
A 2.5 ohm resistor means, 1.2 divided by 2.5 = 0.48 ( 480ma ) or 1.25 divided by 2.5 is 0.5 ( 500 ma ) so using a 2.5 ohm resistor will give you 480-500ma
Remember to configure as a current regulator and not a voltage regulator.
6 x 1 ohm in SERIES = 6 ohms and 1.2 divided by 6 = 0.2A ( 200ma )
5 x 1 ohm in SERIES = 5 ohms and 1.2 divided by 5 = 0.24A ( 240ma )
4 x 1 ohm in SERIES = 4 ohms and 1.2 divided by 4 = 0.3A ( 300ma )
3 x 1 ohm in SERIES = 3 ohms and 1.2 divided by 3 = 0.4A ( 400ma )
2 x 1 ohm in SERIES = 2 ohms and 1.2 divided by 2 = 0.6A ( 600ma )
A single 1 ohm is 1 ohm = 1 ohm and 1.2 divided by 1 = 1.2A
2 x 1 ohm in PARALLEL = 0.5 ohms and 1.2 divided by 0.5 = 2.4A
3 x 1 ohm in PARALLEL = 0.33 ohms and 1.2 divided by 0.33 = 3.6A
4 x 1 ohm in PARALLEL = 0.25 ohms and 1.2 divided by 0.25 = 4.8A
I explained how it works here >
https://laserpointerforums.com/threads/blue-laser-74w-nubm35-portabl.109785/post-1615029
