A Quick Breakdown
- GBB/VSR spec bucking
- 70 degree hardness NBR rubber
- Over-Chambering prevention design
- Pronounced two-point of contact pressure patch
- Alignment fin
 
Which Degree of Crimson GBB Hop-Up Bucking Should You Pick?
The 70 degree Crimson, found here, is a great choice for those of you shooting heavy BBs between 0.30g - 0.45g+, or those of you with very heavy/steel bolts. The added rubber strength of a higher degree helps combat over-chambering that much more. 

The 65 degree Crimson is a great all rounder for BB weights between 0.28g - 0.40g. It works great in a wider variety of GBBs, especially those with over-chambering.

Both will yield great results regardless. 

A material called NBR is used to construct the Crimson bucking. This rubber is far more durable than your traditional silicone rubber, which could easily tear during installation. 
The Crimson Bucking will be sure to last you a good while before any necessary maintenance. 

The Crimson GBB hop-up bucking has been tested over a wide range of GBBs.
 
Below is a small list of just some of the platforms;
 
VFC AR/M4 variants (including the new EVO series)
VFC AK variants
VFC MP5 variants
VFC G36
VFC G3
VFC M249
VFC PPSH
VFC MP7
VFC G-Series pistols
Tokyo Marui GBB Pistols (a wide range from G17s, to Hi-Capas, to USPs etc)
Tokyo Marui MWS M4 variants
APFG MCX/MPX variants
APFG XM7
GHK M4/416 Variants
Vipertech M4 (using Meach Hop-Up chambers)
Maruyama ISO-556
 
*Disclaimer; Results will vary depending on GBB used, recoiling mass, recoil spring strength, intended weigh used, ambient temperature, fps/joule targets, and gas pressure used. 
Further degrees of hop-up buckings, both softer and harder, coming soon to satisfy all use cases.