This has been reworded a little for all you guys with a perverted mind.
Systema, Guarder, Madbull etc. What do you use and why?
I have yet to find a broad and objective comparison of buckings online. I've discussed this with the Evike's air-techs and we've all come to the conclusion that buckings are one of the most subjective and biased upgrade parts.
I'll cut through the brand bull and offer this simple bit of advice. Keep your bucking until you need to change it (like if your BB's are going all over the place.) Brand doesn't matter as much as hardness and newness.
The Long Story
Buckings are the small rubber sleeves that cap the inside end of your inner barrel. The hop-up unit pushes a spacer down, pushing part of the bucking through the hole in the base of your inner barrel. This creates the friction surface that makes your BB spin backward as it shoots past it: the hop-up effect.
Buckings serve a secondary function of holding a single BB in place in your in front of your air nozzle. Otherwise your entire magazine would empty out into your barrel. Double feeding can be caused by worn out or damaged buckings.
Buckings are made of rubber or silicone or a mixture of both and come in various hardness/stiffness ratings. Clear or blue buckings are silicone. Black are usually rubber but can be silicone made to rubber-like specs. Silicone is softer than rubber. But rubber is more durable and lasts longer than silicone.
Softer is generally better for consistency which means better accuracy.
But AEG's that shoot above 375fps may require harder buckings. If you have to dial your hop-up all the way, that's a sign that you need a harder bucking or that your bucking is wearing out. Also if your BB's are very inconsistent or their trajectory changes wildly between semi and full auto, that's a sign you need to change your bucking.
Some guys change their buckings every few months. Me, I've had the same Guarder 70% bucking in my AEG for over 2 years. Anybody who's bit hit by me obviously knows I've had no problems with accuracy.
Don't forget new buckings do need a little breaking in. New ones are covered in a thin coating of silicone lube to protect from from oxidation and cracking. That coating needs to be worn away by BB's (or cleaned off with detergent and warm water and a dry Q-tip before installation).
Lubing your barrel or hop-up with silicone oil to clean it, will also adversely change your bucking's performance until this lube is worn away.